fix(activate): Don't set PYTHONEXECUTABLE

[arrdem]

Paired with @alexeagle. It used to be before recent improvements to runfiles tree identification and the venv shim that we relied on the activate script to set PYTHONEXECUTABLE. This proved to be unreliable, in part because many tools just don't evaluate the activate script.

Setting the PYTHONEXECUTABLE without also setting PYTHONHOME breaks non-Bazel Python invocations and so is a bug. Furthermore we didn't save/restore or more importantly unset PYTHONEXECUTABLE during deactivate, which broke shell environments.

Just don't set this.

Fixes #671.

Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: yes

Fixed #671, activate scripts no longer poison the shell env.

Test plan

• Manual testing

[aspect-workflows]

test-os:linux-bzl:8 (Test)

2 test targets passed

Targets

//py/tests/py_venv_image_layer:my_app_amd64_layers_test [k8-fastbuild]           61ms
//py/tests/py_venv_image_layer:my_app_arm64_layers_test [k8-fastbuild]           68ms

Total test execution time was 129ms. 36 tests (94.7%) were fully cached saving 59s.

---

test-os:linux-bzl:latest (Test)

All tests were cache hits

38 tests (100.0%) were fully cached saving 60s.

---

test-os:linux-bzl:8 (Test)

e2e

All tests were cache hits

10 tests (100.0%) were fully cached saving 7s.

---

test-os:linux-bzl:latest (Test)

e2e

All tests were cache hits

10 tests (100.0%) were fully cached saving 7s.

---

test-os:linux-bzl:8 (Test)

examples/uv_pip_compile

All tests were cache hits

1 test (100.0%) was fully cached saving 335ms.

---

test-os:linux-bzl:latest (Test)

examples/uv_pip_compile

All tests were cache hits

1 test (100.0%) was fully cached saving 335ms.