fix(venv): Use the interpreter to abspath

[arrdem]

Identified in manual testing of v1.6.0-rc0.

Because of overly conservative removals of realpath in #579, an issue is exposed where $VIRTUAL_ENV would be an un-resolved relative path both under Bazel and more significantly once a user activates a linked venv. This isn't so bad for binaries which usually don't chdir, but it's a problem for shells.

Since MacOS doesn't ship a realpath which can be configured to ignore symlinks, we can't just realpath the runfiles dir or the virtualenv home. But once we've configured a Python interpreter what we can do is use os.path.abspath. Unlike realpath, abspath does not attempt to resolve symlink path segments. It just uses normpath to eliminate relative path segments. This allows us to compute an absolute path in a portable way, once we get an appropriate interpreter up and running.

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Changes are visible to end-users: no

Test plan

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

❯ bazel run //examples/py_binary:py_binary.venv
INFO: Analyzed target //examples/py_binary:py_binary.venv (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //examples/py_binary:py_binary.venv up-to-date:
  bazel-bin/examples/py_binary/py_binary.venv
INFO: Elapsed time: 0.785s, Critical Path: 0.02s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
INFO: Running command line: bazel-bin/examples/py_binary/py_binary.venv
Linking: /private/var/tmp/_bazel_arrdem/93bfea6cdc1153cc29a75400cd38823a/execroot/aspect_rules_py/bazel-out/darwin_arm64-fastbuild/bin/examples/py_binary/.py_binary.venv -> /Users/arrdem/Documents/work/aspect/rules_py/examples/py_binary/.py_binary.venv

To activate the virtualenv run:
    source /Users/arrdem/Documents/work/aspect/rules_py/examples/py_binary/.py_binary.venv/bin/activate

Link is up to date!

❯ source /Users/arrdem/Documents/work/aspect/rules_py/examples/py_binary/.py_binary.venv/bin/activate

❯ env | grep -e PYTHON -e RUNFILES -e VIRTUAL -e VENV | sort
PYTHONEXECUTABLE=/Users/arrdem/Documents/work/aspect/rules_py/examples/py_binary/.py_binary.venv/bin/python
PYTHONHOME=/private/var/tmp/_bazel_arrdem/93bfea6cdc1153cc29a75400cd38823a/execroot/aspect_rules_py/bazel-out/darwin_arm64-fastbuild/bin/examples/py_binary/py_binary.venv.runfiles/python_toolchain_aarch64-apple-darwin
RUNFILES_DIR=/private/var/tmp/_bazel_arrdem/93bfea6cdc1153cc29a75400cd38823a/execroot/aspect_rules_py/bazel-out/darwin_arm64-fastbuild/bin/examples/py_binary/py_binary.venv.runfiles
RUNFILES_MANIFEST_FILE=/private/var/tmp/_bazel_arrdem/93bfea6cdc1153cc29a75400cd38823a/execroot/aspect_rules_py/bazel-out/darwin_arm64-fastbuild/bin/examples/py_binary/py_binary.venv.runfiles/MANIFEST
RUNFILES_REPO_MAPPING=/private/var/tmp/_bazel_arrdem/93bfea6cdc1153cc29a75400cd38823a/execroot/aspect_rules_py/bazel-out/darwin_arm64-fastbuild/bin/examples/py_binary/py_binary.venv.runfiles/_repo_mapping
VIRTUAL_ENV_DISABLE_PROMPT=1
VIRTUAL_ENV=/Users/arrdem/Documents/work/aspect/rules_py/examples/py_binary/.py_binary.venv

[aspect-workflows]

Test

2 test targets passed

Targets

//py/tests/py_venv_image_layer:my_app_amd64_layers_test [k8-fastbuild]                 30ms
//py/tests/py_venv_image_layer:my_app_arm64_layers_test [k8-fastbuild]                 64ms

Total test execution time was 94ms. 38 tests (95.0%) were fully cached saving 1m 9s.

[github-actions]

e2e/use_release folder: LCOV of commit 716c463 during CI #1864

Summary coverage rate:
  lines......: 100.0% (2 of 2 lines)
  functions..: 100.0% (1 of 1 function)
  branches...: no data found

Files changed coverage rate: n/a