Fix touch file creation to update timestamps in build/symlink.py

The old code used open(path, 'w') which truncates existing files to 0
size.  This CL makes the "touch" option more similar to the "touch"
command: it will create the file if it doesn't exist, or updating the
existing file's mtime.

The "touch" option for the symlink template doesn't appear to be used,
so this shouldn't affect any existing code, however it's required for
rolling the instrumented_libraries which now uses "touch".

R=thestig

Change-Id: I6f000899ef00fbb88c1c4ee21456bb8c28e8adc1
Bug: 343086053, 333609800
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5594286
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1309691}

• build/symlink.py[diff]