Update mac_build_instructions.md to avoid python PATH issue

Python is unable to find cipd if the path to depot_tools in PATH
is relative or uses shell tokens like '~'.

[email protected]

Bug: 952865
Change-Id: I1b17921f84862e515108ac449314f8497b9cbb84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1571488
Reviewed-by: Nico Weber <[email protected]>
Commit-Queue: Gabriel Charette <[email protected]>
Auto-Submit: Gabriel Charette <[email protected]>
Cr-Commit-Position: refs/heads/master@{#653608}
1 file changed