Publish images for all commits on release branches (#63985)

In order to run nightly vulnerability scans of Sourcegraph releases, we
need to publish a new set of images whenever the release branch is
pushed to.

Previously, this was implemented in
https://github.com/sourcegraph/sourcegraph/pull/63379 but with RFC 795
the release branch format changed from 5.5.1234 to 5.5.x.

This PR updates the regex to catch this new format.

The end result of this is that whenever Buildkite runs on a branch
matching `\d.\d.x`, it will push images to the
`us.gcr.io/sourcegraph-dev/gitserver` registry with the tag
`$branch-insiders`.

I've also tagged this PR for backport as we want it on the current patch
release branch 5.5.x :)

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## Test plan

- Test buildkite run on branch `will-0.0.x` (with modified regex to
match that branch)
https://buildkite.com/sourcegraph/sourcegraph/builds/283608

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## Changelog

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dev/ci/push_all.sh

@@ -119,7 +119,7 @@ elif [[ "$BUILDKITE_BRANCH" =~ ^[0-9]+\.[0-9]+$ ]]; then
   # format introduced by https://github.com/sourcegraph/sourcegraph/pull/48050
   # by release branch deployments.
   push_prod=true
-elif [[ "$BUILDKITE_BRANCH" =~ ^[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
+elif [[ "$BUILDKITE_BRANCH" =~ ^[0-9]+\.[0-9]+\.(x|[0-9]+)$ ]]; then
   # Patch release builds only need to be pushed to internal registries.
   push_prod=false
   dev_tags+=("$BUILDKITE_BRANCH-insiders")

dev/ci/runtype/runtype.go

@@ -141,7 +141,7 @@ func (t RunType) Matcher() *RunTypeMatcher {
 		}
 	case PatchReleaseBranch:
 		return &RunTypeMatcher{
-			Branch:       `^[0-9]+\.[0-9]+\.[0-9]+$`,
+			Branch:       `^[0-9]+\.[0-9]+\.(?:x|[0-9]+)$`,
 			BranchRegexp: true,
 		}
 	case BextReleaseBranch: