This change improves the detection logic and bootstrap for GCE keyless signing by also supporting a standard environment variable and library used by gcp sdks
The previous logic just looked for a file to figure out if its on gce or not while this change adds to that by using a metadata server environment variable.
This is useful in testing and provides ability to acquire gcp id_tokens in various environments (off the shelf kubernetes, using TPM (i know you can fulfill the latter directly)
Signed-off-by: sal rashid <salrashid123@gmail.com>
3274cf98c6