Read env value from ENV_DIR instead of env directly

[gerynugrh]

From the API Documentation, during compile phase, build pack should have read env value from $ENV_DIR, e.g. $ENV_DIR/GOOGLE_CREDENTIALS.

This commit changes the workflow of the build pack to match API Documentation.

[elishaterada]

@dmathieu looping you in on this one

[dmathieu]

We don't want to read environment variables at build time. We want to read it at boot time.
So the content of the .sh script should be echo ${CREDENTIALS@Q} > /app/google-credentials.json, with the variable name used, not its value.

Your change would store the value at build time. So changing the config variables would require a new build.

[sineraves]

Both approaches are reading the environment variable at compile time. To read the current value of $GOOGLE_CREDENTIALS during startup, it needs to be escaped when writing the profile.d script. Please see #5