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

[benjaminp]

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.

[tjgq]

What a tricky bug! Thanks for the fix.