Fix writable sandbox path

The reverts in 3fddc7f accidentally replaced `sandboxExecRoot` with `execRoot` in the function that computes writable directories for sandbox spawn runners.

This issue was observed in MobileNativeFoundation/rules_xcodeproj#3029.

Closes #22443.

PiperOrigin-RevId: 635383877
Change-Id: I3d74bf78cf97ecd267ba90b68cf1b715850aba27

src/main/java/com/google/devtools/build/lib/sandbox/AbstractSandboxSpawnRunner.java

@@ -372,7 +372,7 @@ protected ImmutableSet<Path> getWritableDirs(Path sandboxExecRoot, Map<String, S
     // On Windows, sandboxExecRoot is actually the main execroot. We will specify
     // exactly which output path is writable.
     if (OS.getCurrent() != OS.WINDOWS) {
-      writablePaths.add(execRoot);
+      writablePaths.add(sandboxExecRoot);
     }
 
     String testTmpdir = env.get("TEST_TMPDIR");