[SplitBuild] Remove persistent install / dxc-dist directories

.github/workflows/build-and-test-callable.yaml

@@ -266,6 +266,8 @@ jobs:
         shell: bash
         run: |
             set -euxo pipefail
+            # Prevent stale extraction dirs from previous runs from persisting            
+            rm -rf "$GITHUB_WORKSPACE/install" "$GITHUB_WORKSPACE/dxc-dist"
             cd $GITHUB_WORKSPACE/llvm-project/build
             ninja install-distribution install-offload-tools install-offload-test-suite
             # Stage DXC into a portable bin/+lib/ tree at dxc-dist.
@@ -358,6 +360,7 @@ jobs:
         run: |
             set -euxo pipefail
             cd $GITHUB_WORKSPACE
+            rm -rf install dxc-dist
             mkdir -p install dxc-dist
             # Pipe the archive in via stdin and use a relative -C path so
             # tar doesn't try to parse the Windows drive letter or treat

docs/offload-distribution.md

@@ -131,6 +131,7 @@ Extract both prefixes, then run the configure script with `--dxc-path`
 pointing at the DXC binary in its prefix:
 
 ```
+rm -rf install dxc-dist
 mkdir install dxc-dist
 tar xf llvm-prefix.tar -C install
 tar xf dxc-prefix.tar  -C dxc-dist
@@ -141,6 +142,13 @@ python configure-test-suite.py \
     --dxc-path ../../../dxc-dist/bin/dxc[.exe]
 ```
 
+> **Always extract into freshly-cleaned prefixes.** `tar xf` overlays files
+> onto whatever already exists and never removes orphans, so on a reused
+> working directory (e.g. a persistent self-hosted runner) stale `.test`
+> files from a previous build would survive and be run by lit. The `rm -rf`
+> above guarantees lit sees only the tests in this tarball. The build runner
+> likewise wipes its `install`/`dxc-dist` prefixes before installing.
+
 This emits a fully-substituted `run/test/<suite>/lit.site.cfg.py`.
 
 `dxc[.exe]` finds its runtime libraries automatically: on Windows the