ci: skip cache warmup if the "previous" commit uses a different toolchain (leanprover-community#37653)

leanprover-community#37564 changed `cache` so that it looks for a bundled `leantar` rather than downloading a copy. This caused the "warmup cache" step introduced in leanprover-community#36623 to break since the "previous" commit being fetched could be using an older toolchain.

We fix this by simply skipping getting the cache for the previous commit if the toolchain differs. It's likely that the oleans are all invalidated anyways.

Prepared with claude code.

Author: bryangingechen

.github/workflows/build_template.yml

@@ -310,13 +310,19 @@ jobs:
             # cf. https://leanprover.zulipchat.com/#narrow/channel/287929-mathlib4/topic/Mathlib.20has.20moved.20to.20the.20new.20module.20system/near/563452000
             echo "Warming up cache using previous commit: $PREV_SHA (source: $PREV_SHA_SOURCE)"
             if git cat-file -e "$PREV_SHA^{commit}" 2>/dev/null || git fetch --no-tags --depth=1 origin "$PREV_SHA"; then
-              git checkout "$PREV_SHA"
-              ../tools-branch/.lake/build/bin/cache get
-              # Run again with --repo, to ensure we actually get the oleans.
-              ../tools-branch/.lake/build/bin/cache --repo="$CACHE_REPO" get
-
-              echo "Switching back to branch head"
-              git checkout "$ORIG_SHA"
+              # Skip warmup if the previous commit uses a different toolchain
+              PREV_TOOLCHAIN=$(git show "$PREV_SHA:lean-toolchain" 2>/dev/null || true)
+              if [[ "$PREV_TOOLCHAIN" != "$(cat lean-toolchain)" ]]; then
+                echo "Previous commit $PREV_SHA uses a different toolchain ($PREV_TOOLCHAIN); skipping warmup."
+              else
+                git checkout "$PREV_SHA"
+                ../tools-branch/.lake/build/bin/cache get
+                # Run again with --repo, to ensure we actually get the oleans.
+                ../tools-branch/.lake/build/bin/cache --repo="$CACHE_REPO" get
+
+                echo "Switching back to branch head"
+                git checkout "$ORIG_SHA"
+              fi
             else
               echo "Could not fetch $PREV_SHA; skipping parent warmup cache fetch."
             fi