Override cipd install mode for runhooks

Currently cipd packages listed in src/DEPS are being created and
installed in symlink mode (see bug for more info). This CL adds `$OverrideInstallMode copy` to the ensure files for cipd packages installed during gclient runhooks. `$OverrideInstallMode` was
implemented here, https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3124202,
so that every cipd package can be installed directly to the installation
target, instead of being symlinked to an executable inside the cache/builder/.cipd dir.

The added `$OverrideInstallMode copy` can be removed once all cipd
packages themselves or the default mode is updated to be created in
copy mode.

Bug: 1329641
Change-Id: I68dc718e20f193c384d005c9b6b87603753943d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3673698
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Stephanie Kim <kimstephanie@google.com>

gclient_scm.py

@@ -1542,7 +1542,10 @@ class CipdRoot(object):
   @contextlib.contextmanager
   def _create_ensure_file(self):
     try:
-      contents = '$ParanoidMode CheckPresence\n\n'
+      contents = '$ParanoidMode CheckPresence\n'
+      # TODO(crbug/1329641): Remove once cipd packages have been updated
+      # to always be created in copy mode.
+      contents += '$OverrideInstallMode copy\n\n'
       for subdir, packages in sorted(self._packages_by_subdir.items()):
         contents += '@Subdir %s\n' % subdir
         for package in sorted(packages, key=lambda p: p.name):

tests/gclient_cipd_smoketest.py

@@ -35,6 +35,7 @@ class GClientSmokeCipd(gclient_smoketest_base.GClientSmokeBase):
     tree.update({
         '_cipd': '\n'.join([
             '$ParanoidMode CheckPresence',
+            '$OverrideInstallMode copy',
             '',
             '@Subdir src/another_cipd_dep',
             'package1 1.1-cr0',
@@ -77,6 +78,7 @@ class GClientSmokeCipd(gclient_smoketest_base.GClientSmokeBase):
     tree.update({
         '_cipd': '\n'.join([
             '$ParanoidMode CheckPresence',
+            '$OverrideInstallMode copy',
             '',
             '@Subdir src/repo12',
             'foo 1.3',
@@ -99,6 +101,7 @@ class GClientSmokeCipd(gclient_smoketest_base.GClientSmokeBase):
     tree.update({
         '_cipd': '\n'.join([
             '$ParanoidMode CheckPresence',
+            '$OverrideInstallMode copy',
             '',
             '@Subdir src/repo12',
             'foo 1.3',
@@ -118,6 +121,7 @@ class GClientSmokeCipd(gclient_smoketest_base.GClientSmokeBase):
     tree.update({
         '_cipd': '\n'.join([
             '$ParanoidMode CheckPresence',
+            '$OverrideInstallMode copy',
             '',
             '@Subdir src/repo12',
             'foo 1.3',