fuchsia: Fix some packaging when run isn't from Crashpad source root
fuchsia: Fix some packaging when run isn't from Crashpad source root

- Use gn --root rather than setting the cwd when running GN so it can
find //.gn.

- Use source-relative path for assets when building the target path, see
for example the failure in:
https://build.chromium.org/p/client.crashpad/builders/crashpad_fuchsia_x64_rel/builds/77/steps/run%20tests/logs/stdio

Bug: crashpad:196
Change-Id: If95636fcb826c22d9d9543cad02f815780621414
Reviewed-on: https://chromium-review.googlesource.com/923436
Commit-Queue: Scott Graham <e134406e1863f3b48b0795831afac5c0595efb36@chromium.org>
Reviewed-by: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>

chromium/crashpad,chromium/crashpad,chromium/crashpad