CLOUDP-301555 - cleanup switch context by running sourcing only once (#4115)

# Summary

- we were sourcing `context.file` 2 times, one before creating
`current_envs` and then when creating `current_envs`. This was because
we were relying on `env` instead of `export -p` which does not support
proper exporting
- additionally replaces all `#!/bin/bash` to `#!/usr/bin/env bash` for
consistency
- exports PATH for the `env -i` command to ensure we have all the
executables running the source
- in the end we only "eval" the export -p output instead of sourcing it,
which only runs export and not the full script (again)

## Proof of Work

```
make switch context=e2e_static_multi_cluster_kind
Switching context to: e2e_static_multi_cluster_kind
Generated env files in /Users/nam.nguyen/projects/ops-manager-kubernetes/.generated:
context.env
context.export.env
context.operator.env
context.operator.export.env
Switched to context "kind-e2e-operator".
Current context: e2e_static_multi_cluster_kind (kubectl context: kind-e2e-operator), namespace=nnguyen-evg-singl
```

## Checklist
- [x] Have you linked a jira ticket and/or is the ticket in the title?
- [x] Have you checked whether your jira ticket required DOCSP changes?
- [x] Have you checked for release_note changes?

## Reminder (Please remove this when merging)
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short as possible
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for clarity:
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Author: nammn

.githooks/pre-commit

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -Eeou pipefail
 

docker/mongodb-enterprise-appdb-database/docker-entrypoint.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -Eeuo pipefail
 
 if [ "${1:0:1}" = '-' ]; then

docker/mongodb-enterprise-init-database/content/probe.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -Eeou pipefail
 
 check_process() {

multi_cluster/setup_multi_cluster_environment.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 aws_region="eu-west-2"
 region="${aws_region}a"

multi_cluster/tools/download_istio.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 set -Eeou pipefail
 
 export VERSION=${VERSION:-1.16.1}

multi_cluster/tools/install_istio.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -eux
 

multi_cluster/tools/install_istio_central.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -eux
 

public/architectures/setup-multi-cluster/setup-istio/install_istio_separate_network.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # This script is an adjusted version of the official Istio guide:
 # https://istio.io/latest/docs/setup/install/multicluster/multi-primary_multi-network/

public/tools/multicluster/install_istio_separate_network.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -eux
 

public/tools/multicluster/sign.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -euo pipefail