The GitHub Actions provider decoded the response body without checking the
status code, so a failing OIDC endpoint surfaced as a raw json error:
fetching ambient OIDC credentials: invalid character 'u' looking for
beginning of value
Check the status code first and report it along with a truncated body, and
wrap decode failures with the provider name:
fetching ambient OIDC credentials: identity token request to provider
github-actions failed with status 502 Bad Gateway: "upstream connect
error or disconnect/reset before headers"
Fixes#4438
Signed-off-by: Eugene Kalinin <e.v.kalinin@gmail.com>
e9af93ff29