Use consistent commit for update-otel (#40462)

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#### Description

Right now we use 'main' as the commit when updating:

https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/0087046281afca6f7607703841fd8cecbc778dae/Makefile#L5-L6

This is troublesome if there are pushes to main while the update-otel is
running, or just in general since we go via the Go proxy and sometimes
the latest commit has not been chached. This changes things so that we
use a fixed commit from the checkout.

Author: mx-psi

.github/workflows/update-otel.yaml

@@ -20,13 +20,14 @@ jobs:
       - name: Update to latest opentelemetry-collector release and create a PR
         run: |
           exec > >(tee log.out) 2>&1
+          LAST_COMMIT=$(git -C ./opentelemetry-collector/ rev-parse HEAD)
           cd opentelemetry-collector-contrib
           git config user.name opentelemetrybot
           git config user.email [email protected]
           branch="opentelemetrybot/update-otel-$(date +%s)"
           git checkout -b $branch
           make genotelcontribcol
-          make update-otel
+          make update-otel OTEL_STABLE_VERSION=${LAST_COMMIT} OTEL_VERSION=${LAST_COMMIT}
           git push --set-upstream origin $branch  
           gh pr create --base main --title "[chore] Update core dependencies" --body "This PR updates the opentelemetry-collector dependency to the latest release"
         env: