Read GOOGLE_CREDENTIALS during startup, not app compilation

[sineraves]

This pull request escapes the reference to the GOOGLE_CREDENTIALS environment variable when creating the $BUILD_DIR/.profile.d/google-credentials.sh script, so the script ends up containing:

echo ${GOOGLE_CREDENTIALS} > /app/google-credentials.json

This variable is then read during startup when the profile.d script runs and its current contents written to /app/google-credentials.json. (Prior to this change it was being read during compilation, and not with the suggested approach of going via $ENV_DIR).

Tested against heroku-18 stack.

[sineraves]

We've since added the echo ${GOOGLE_CREDENTIALS} > /app/google-credentials.json line to a .profile file in our repository.

It still reads the credentials from the $GOOGLE_CREDENTIALS environment variable, but removes the need for using a Buildpack.

I'll leave this PR up in case it's useful for anyone who doesn't want to use that approach.

[playerla]

The fix work for me