tweak(py_venv): Don't modifying shell flags in activate (#669)

### Changes are visible to end-users: yes

When creating a virtual environment, the `activate` script no longer
modifies bash flags. These flags were added in
#576 (v1.6.0) (note, they
don't appear in the scripts that Python's `venv` module generates).

<!-- If no, please delete this section. -->

- Searched for relevant documentation and updated as needed: yes
(behaviour is not documented)
- Breaking change (forces users to change their own code or config):no
- Suggested release notes appear below: yes/no

### Test plan

- Manual testing; please provide instructions so we can reproduce:

Potentially, users could have some tests running the following commands.
The new flags introduced in
#576 broke this kind of
tests:

```sh
#!/usr/bin/env bash
set -euo pipefail

# Create a virtualenvironment
bazel run //:venv

# Activate it (previous it was setting +e flag)
source .venv/bin/activate

# Some failing test -- users expects the script to end and fail here
false

# Some other command that doesn't fail
true

# !!!! Script returns 0 !!!!
```

Author: jgsogo

py/tests/py_venv_image_layer/my_app_amd64_layers_listing.yaml

@@ -2510,7 +2510,7 @@ files:
   - drwxr-xr-x  0 0      0           0 Jan  1  2023 ./py/tests/py_venv_image_layer/my_app_bin.runfiles/aspect_rules_py/py/tests/py_venv_image_layer/
   - drwxr-xr-x  0 0      0           0 Jan  1  2023 ./py/tests/py_venv_image_layer/my_app_bin.runfiles/aspect_rules_py/py/tests/py_venv_image_layer/.my_app_bin/
   - drwxr-xr-x  0 0      0           0 Jan  1  2023 ./py/tests/py_venv_image_layer/my_app_bin.runfiles/aspect_rules_py/py/tests/py_venv_image_layer/.my_app_bin/bin/
-  - -rwxr-xr-x  0 0      0        2853 Jan  1  2023 ./py/tests/py_venv_image_layer/my_app_bin.runfiles/aspect_rules_py/py/tests/py_venv_image_layer/.my_app_bin/bin/activate
+  - -rwxr-xr-x  0 0      0        2811 Jan  1  2023 ./py/tests/py_venv_image_layer/my_app_bin.runfiles/aspect_rules_py/py/tests/py_venv_image_layer/.my_app_bin/bin/activate
   - -rwxr-xr-x  0 0      0      870664 Jan  1  2023 ./py/tests/py_venv_image_layer/my_app_bin.runfiles/aspect_rules_py/py/tests/py_venv_image_layer/.my_app_bin/bin/python
   - -rwxr-xr-x  0 0      0      870664 Jan  1  2023 ./py/tests/py_venv_image_layer/my_app_bin.runfiles/aspect_rules_py/py/tests/py_venv_image_layer/.my_app_bin/bin/python3
   - -rwxr-xr-x  0 0      0      870664 Jan  1  2023 ./py/tests/py_venv_image_layer/my_app_bin.runfiles/aspect_rules_py/py/tests/py_venv_image_layer/.my_app_bin/bin/python3.9

py/tests/py_venv_image_layer/my_app_arm64_layers_listing.yaml

@@ -2491,7 +2491,7 @@ files:
   - drwxr-xr-x  0 0      0           0 Jan  1  2023 ./py/tests/py_venv_image_layer/my_app_bin.runfiles/aspect_rules_py/py/tests/py_venv_image_layer/
   - drwxr-xr-x  0 0      0           0 Jan  1  2023 ./py/tests/py_venv_image_layer/my_app_bin.runfiles/aspect_rules_py/py/tests/py_venv_image_layer/.my_app_bin/
   - drwxr-xr-x  0 0      0           0 Jan  1  2023 ./py/tests/py_venv_image_layer/my_app_bin.runfiles/aspect_rules_py/py/tests/py_venv_image_layer/.my_app_bin/bin/
-  - -rwxr-xr-x  0 0      0        2853 Jan  1  2023 ./py/tests/py_venv_image_layer/my_app_bin.runfiles/aspect_rules_py/py/tests/py_venv_image_layer/.my_app_bin/bin/activate
+  - -rwxr-xr-x  0 0      0        2811 Jan  1  2023 ./py/tests/py_venv_image_layer/my_app_bin.runfiles/aspect_rules_py/py/tests/py_venv_image_layer/.my_app_bin/bin/activate
   - -rwxr-xr-x  0 0      0      743120 Jan  1  2023 ./py/tests/py_venv_image_layer/my_app_bin.runfiles/aspect_rules_py/py/tests/py_venv_image_layer/.my_app_bin/bin/python
   - -rwxr-xr-x  0 0      0      743120 Jan  1  2023 ./py/tests/py_venv_image_layer/my_app_bin.runfiles/aspect_rules_py/py/tests/py_venv_image_layer/.my_app_bin/bin/python3
   - -rwxr-xr-x  0 0      0      743120 Jan  1  2023 ./py/tests/py_venv_image_layer/my_app_bin.runfiles/aspect_rules_py/py/tests/py_venv_image_layer/.my_app_bin/bin/python3.9

py/tools/py/src/activate.tmpl

@@ -1,7 +1,5 @@
 # Adapted from CPython Lib/venv/scripts/common/activate
 
-set -eu -o pipefail
-
 deactivate () {
     # reset old environment variables
     if [ -n "${_OLD_VIRTUAL_PATH:-}" ] ; then
@@ -84,5 +82,3 @@ export PATH
 # Call hash to forget past commands. Without forgetting
 # past commands the $PATH changes we made may not be respected
 hash -r 2> /dev/null
-
-set +eu +o pipefail