bazel: set BAZEL_SH only when msys2 bash exists

[l46983284-cpu]

Prerequisites

• Use conventional PR title: <manifest-name[@version]|chore>: <general summary of the pull request>
• I have read the Contributing Guide

Summary

bazel used env_set to always write BAZEL_SH to $(appdir msys2)/current/usr/bin/bash.exe. If msys2 was only suggested (not installed), users got a non-existent path (#7402).

This PR:

• removes unconditional env_set
• sets BAZEL_SH in installer only when that bash path exists
• removes BAZEL_SH on uninstall
• documents manual override when using another bash (e.g. git)

Closes #7402

Test plan

• bazel.json parses as JSON
• Windows: install without msys2 → no broken BAZEL_SH
• Windows: install with msys2 → BAZEL_SH points at msys2 bash

[l46983284-cpu]

/verify

[github-actions]

All changes look good.

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[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 47b987ac-2456-4f0c-999c-8a09551f7691

📥 Commits

Reviewing files that changed from the base of the PR and between 9806b94 and 0ef150a.

📒 Files selected for processing (1)

• bucket/bazel.json

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Summary by CodeRabbit

• Bug Fixes
  • Improved Bazel installation handling for the BAZEL_SH environment variable.
  • BAZEL_SH is now configured automatically only when the expected MSYS2 Bash executable is available.
  • Added guidance for manually configuring BAZEL_SH when MSYS2 is not installed or an alternative Bash source is used.
  • Uninstallation now reliably removes the configured BAZEL_SH setting.

Walkthrough

Changes

Bazel shell configuration

Layer / File(s)	Summary
Conditional BAZEL_SH setup bucket/bazel.json	Installation notes document the msys2 requirement and issue reference. Installer scripts set BAZEL_SH only when the expected bash.exe exists, otherwise prompting manual configuration; uninstallation removes the recorded variable.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	The PR fixes the msys2-path bug, but it does not implement the issue's requested Get-Command bash resolution.	Set BAZEL_SH from the detected bash path when msys2 is absent, rather than only skipping the variable or requiring manual setup.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and accurately summarizes the main manifest change.
Description check	✅ Passed	The description includes the required summary, prerequisites, issue link, and test plan.
Out of Scope Changes check	✅ Passed	The changes stay focused on BAZEL_SH handling and manifest notes, with no clear unrelated additions.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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