ci: ensure v master is available when trying to check out its commits…

… to build oldv (#21414)

.github/workflows/bootstrapping_ci.yml

@@ -40,7 +40,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
         with:
-          fetch-depth: 10
+          fetch-depth: 0
       - name: Build V
         run: make -j4
       - name: Test bootstrapping (v.c can be compiled and run with -os cross)
@@ -68,6 +68,9 @@ jobs:
           ls -la v vc/v.c
           ls -la v_from_vc v_from_vc_produced_native_v
           ls -la v_from_vc2 v_from_vc_produced_native_v2
+      - name: Ensure V master is available
+        if: github.ref_name != 'master'
+        run: git branch master remotes/origin/master
       - name: Test `v up`
         run: |
           # Derive a commit sha from an older successful fast workflow on master that was able to build V.
@@ -79,7 +82,9 @@ jobs:
             "https://api.github.com/repos/vlang/v/actions/workflows/18477644/runs?branch=master&status=success&per_page=2" \
             | jq -r '.workflow_runs[1].head_sha')
           echo "recent_good_commit=$recent_good_commit"
+          # Build oldv at recent_good_commit.
           ./v run cmd/tools/oldv.v -v $recent_good_commit
           cd ~/.cache/oldv/v_at_$recent_good_commit
+          # Test updating
           ./v version && ./v -v up && ./v version
           ./v -o v2 cmd/v && ./v2 -o v3 cmd/v