Ensure runfiles output manifest is gone before making it a link.

After 0763dd0, the test included in this CL fails like this:
```
** test_switch_runfiles_from_enabled_to_disabled *******************************
Computing main repo mapping:
Loading:
Loading: 0 packages loaded
Analyzing: target //:out (1 packages loaded, 0 targets configured)
INFO: Analyzed target //:out (42 packages loaded, 172 targets configured).
INFO: Found 1 target...
[0 / 2] [Prepa] BazelWorkspaceStatusAction stable-status.txt
Target //:out up-to-date:
  bazel-bin/out
INFO: Elapsed time: 0.391s, Critical Path: 0.02s
INFO: 5 processes: 4 internal, 1 local.
INFO: Build completed successfully, 5 total actions
Computing main repo mapping:
Loading:
Loading: 0 packages loaded
WARNING: Build option --enable_runfiles has changed, discarding analysis cache (this can be expensive, see https://bazel.build/advanced/performance/iteration-speed).
Analyzing: target //:out (1 packages loaded, 0 targets configured)
INFO: Analyzed target //:out (1 packages loaded, 173 targets configured).
INFO: Found 1 target...
[0 / 3] [Prepa] BazelWorkspaceStatusAction stable-status.txt
ERROR: workspace/BUILD:6:8: Executing genrule //:g failed: java.io.IOException: execroot/main/bazel-out/k8-opt-exec-ST-e846b08c7501/bin/cmd.runfiles/MANIFEST (File exists)
Target //:out failed to build
Use --verbose_failures to see the command lines of failed build steps.
INFO: Elapsed time: 0.177s, Critical Path: 0.01s
INFO: 4 processes: 4 internal.
ERROR: Build did NOT complete successfully
```
This is because `FileSystemUtils.copyFile` ensured that the target was removed before writing to the target while `Path.createSymbolicLink` does not. Insert a `delete()` call to fix the problem.

Also, rename `copyManifest` to `linkManifest` for accuracy.

Closes #19382.

PiperOrigin-RevId: 562519152
Change-Id: I8683f84daaa6f827a3022fead50687e4d2eb34d9

src/main/java/com/google/devtools/build/lib/exec/RunfilesTreeUpdater.java

@@ -156,7 +156,7 @@ private void updateRunfilesTree(
 
     switch (runfileSymlinksMode) {
       case SKIP:
-        helper.copyManifest();
+        helper.linkManifest();
         break;
       case EXTERNAL:
         helper.createSymlinksUsingCommand(execRoot, binTools, env, outErr);

src/main/java/com/google/devtools/build/lib/exec/SymlinkTreeHelper.java

@@ -130,12 +130,13 @@ public void clearRunfilesDirectory() throws ExecException {
     }
   }
 
-  /** Copies the input manifest to the output manifest. */
-  public void copyManifest() throws ExecException {
+  /** Links the output manifest to the input manifest. */
+  public void linkManifest() throws ExecException {
     // Pretend we created the runfiles tree by symlinking the output manifest to the input manifest.
     Path outputManifest = getOutputManifest();
     try {
       symlinkTreeRoot.createDirectoryAndParents();
+      outputManifest.delete();
       outputManifest.createSymbolicLink(inputManifest);
     } catch (IOException e) {
       throw new EnvironmentalExecException(e, Code.SYMLINK_TREE_MANIFEST_LINK_IO_EXCEPTION);

src/main/java/com/google/devtools/build/lib/exec/SymlinkTreeStrategy.java

@@ -107,7 +107,7 @@ public void createSymlinks(
           // Skyframe does not clear the directory for us.
           var helper = createSymlinkTreeHelper(action, actionExecutionContext);
           helper.clearRunfilesDirectory();
-          helper.copyManifest();
+          helper.linkManifest();
         } else if (action.getRunfileSymlinksMode() == RunfileSymlinksMode.INTERNAL
             && !action.isFilesetTree()) {
           try {

src/test/shell/bazel/runfiles_test.sh

@@ -239,4 +239,26 @@ EOF
   [[ -f bazel-bin/world.runfiles/MANIFEST ]] || fail "expected output manifest world to exist"
 }
 
+function test_switch_runfiles_from_enabled_to_disabled {
+    echo '#!/bin/bash' > cmd.sh
+    chmod 755 cmd.sh
+    cat > BUILD <<'EOF'
+sh_binary(
+  name = "cmd",
+  srcs = ["cmd.sh"],
+  data = glob(["data-*"]),
+)
+genrule(
+  name = "g",
+  cmd = "$(location :cmd) > $@",
+  outs = ["out"],
+  tools = [":cmd"],
+)
+EOF
+
+    bazel build --spawn_strategy=local --nobuild_runfile_links //:out
+    touch data-1
+    bazel build --spawn_strategy=local --nobuild_runfile_links --enable_runfiles=false //:out
+}
+
 run_suite "runfiles tests"