GitHub ci(windows): speed up initializing Git for Windows' minimal SD…

…K again

It used to be the case that initializing the minimal SDK (i.e. a
radically slimmed-down subset of Git for Windows' development
environment intended to perform the CI builds and little else) took
a bit over one minute, would then be cached, and subsequent jobs would
take at most half a dozen seconds to initialize said minimal SDK.

It is important that this step is fast because we have to run the test
suite in parallel, in a set of matrix jobs, to offset the slowness of
the shell-based test suite, and each and every job has to initialize the
very same minimal SDK.

While it may sound as if parallelizing the jobs might only waste the
generously-provided build minutes but at least the _wallclock_ time
would pass quick, in reality it matters a lot: Frequently Git for
Windows' or GitGitGadget PRs get stuck waiting for quite a while before
CI builds start because other PRs' builds still spend substantial
amounts of time to run, blocking due to the concurrency limit being
reached.

Since 91839a8 (ci: create script to set up Git for Windows SDK,
2024-10-09), the situation has worsened: every job that requires the
minimal Git for Windows SDK spends roughly two-and-a-half minutes doing
so.

With the switch away from the GitHub Action `setup-git-for-windows-sdk`,
we incurred more downsides:

- It is no longer possible for said Action to fix problems independently
  from the Git repository, e.g. when new rules about GitHub Actions
  require changes in the way the minimal SDK is initialized.

- The minimal SDK was installed specifically outside of the worktree so
  as not to clutter it nor incur an additional cost to verify that the
  worktree is clean.

Therefore, even if it would be nice to have a shared process between
GitHub and GitLab based CI builds, let's switch the GitHub-based CI back
to the tried-and-tested `setup-git-for-windows-sdk` Action.

This commit partially reverts 91839a8 (ci: create script to set up
Git for Windows SDK, 2024-10-09).

Signed-off-by: Johannes Schindelin <[email protected]>

.github/workflows/main.yml

@@ -113,15 +113,13 @@ jobs:
       cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
     - uses: actions/checkout@v4
-    - name: setup SDK
-      shell: powershell
-      run: ci/install-sdk.ps1
+    - uses: git-for-windows/setup-git-for-windows-sdk@v1
     - name: build
-      shell: powershell
+      shell: bash
       env:
         HOME: ${{runner.workspace}}
         NO_PERL: 1
-      run: git-sdk/usr/bin/bash.exe -l -c 'ci/make-test-artifacts.sh artifacts'
+      run: . /etc/profile && ci/make-test-artifacts.sh artifacts
     - name: zip up tracked files
       run: git archive -o artifacts/tracked.tar.gz HEAD
     - name: upload tracked files and build artifacts
@@ -149,12 +147,10 @@ jobs:
     - name: extract tracked files and build artifacts
       shell: bash
       run: tar xf artifacts.tar.gz && tar xf tracked.tar.gz
-    - name: setup SDK
-      shell: powershell
-      run: ci/install-sdk.ps1
+    - uses: git-for-windows/setup-git-for-windows-sdk@v1
     - name: test
-      shell: powershell
-      run: git-sdk/usr/bin/bash.exe -l -c 'ci/run-test-slice.sh ${{matrix.nr}} 10'
+      shell: bash
+      run: . /etc/profile && ci/run-test-slice.sh ${{matrix.nr}} 10
     - name: print test failures
       if: failure() && env.FAILED_TEST_ARTIFACTS != ''
       shell: bash