build: use copy mode for xcode_binaries cipd package

Largest file is 11MB in xcode_binaries cipd package, so I think copying
these files fine.

symlinks point to cipd cache do not work correctly on swarming. This
fixes that by using copy.

Bug: 988079
Change-Id: I00861c59d637d5931fbf6aa0f7b169b1e80ebe7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1777846
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692051}

build/xcode_binaries.yaml

@@ -35,3 +35,6 @@ data:
   - file: Contents/Resources/English.lproj/License.rtf
   - file: Contents/Resources/LicenseInfo.plist
   - file: Contents/version.plist
+# llvm-size and size are run on swarming, those are symlinked to out of
+# isolated tree and produce invdalid symlink if we don't use copy mode here.
+install_mode: copy