Make branch prioritisation more general with a sorting function

The sorting key sorts in the following priority order:
1. First release branches
2. Next branches that create packages
3. Finally, first come, first serve.

This sort key has the added benefit of ensuring that for equal priority
build requests, the one that was first in queue is the first picked up.
Buildbot documentation does not make any guarantees that the initial
requests list is sorted by submit time.

utils.py

@@ -281,15 +281,19 @@ def fnmatch_any(branch: str, patterns: list[str]) -> bool:
 
 
 # Priority filter based on saved package branches
-def nextBuild(builder: Builder,
-              requests: list[BuildRequest]) -> str:
-    for r in requests:
-        if fnmatch_any(r.sources[""].branch, releaseBranches):
-            return r
-    for r in requests:
-        if fnmatch_any(r.sources[""].branch, savedPackageBranches):
-            return r
-    return requests[0]
+def nextBuild(builder: Builder, requests: list[BuildRequest]) -> str:
+    def build_request_sort_key(request: BuildRequest):
+        branch = request.sources[""].branch
+        # Booleans are sorted False first.
+        # Priority is given to releaseBranches, savePackageBranches
+        # then it's first come, first serve.
+        return (
+            not fnmatch_any(branch, releaseBranches),
+            not fnmatch_any(branch, savedPackageBranches),
+            request.getSubmitTime(),
+        )
+
+    return sorted(requests, build_request_sort_key)[0]
 
 
 def canStartBuild(