Ignore GH Action code flow PRs in PRFinder. (#1482)

The Arcade inter-branch merge GH action creates merge commits with a
differently formatted message (ex. `[automated] Merge branch
'prerelease' => 'main' (#8039)`)

Tested by generating PR list between Roslyn main and Roslyn
release/dev18.0:
*Before*:
```md
### Changes from [main](dotnet/roslyn@main) to [release/dev18.0](dotnet/roslyn@release/dev18.0):
[View Complete Diff of Changes](https://github.com/dotnet/roslyn/compare/main...release/dev18.0?w=1)
- [[automated] Merge branch 'main' => 'release/dev18.0' (77445)](dotnet/roslyn#77445)
- [[automated] Merge branch 'main' => 'release/dev18.0' (77441)](dotnet/roslyn#77441)
- [[automated] Merge branch 'main' => 'release/dev18.0' (77431)](dotnet/roslyn#77431)
- [[automated] Merge branch 'main' => 'release/dev18.0' (77409)](dotnet/roslyn#77409)
- [[automated] Merge branch 'main' => 'release/dev18.0' (77374)](dotnet/roslyn#77374)
- [[automated] Merge branch 'main' => 'release/dev18.0' (77292)](dotnet/roslyn#77292)
- [[automated] Merge branch 'main' => 'release/dev18.0' (77226)](dotnet/roslyn#77226)
- [[automated] Merge branch 'main' => 'release/dev18.0' (77153)](dotnet/roslyn#77153)
- [Update Versions.props for Dev18 (77138)](dotnet/roslyn#77138)
```

*After*:
```md
### Changes from [main](dotnet/roslyn@main) to [release/dev18.0](dotnet/roslyn@release/dev18.0):
[View Complete Diff of Changes](https://github.com/dotnet/roslyn/compare/main...release/dev18.0?w=1)
- [Update Versions.props for Dev18 (77138)](dotnet/roslyn#77138)
```

Author: JoeRobich

src/dotnet-roslyn-tools/PRFinder/Hosts/GitHub.cs

@@ -55,7 +55,8 @@ public bool ShouldSkip(Commit commit, ref bool mergePRFound)
         }
 
         // Exclude merge commits from auto code-flow PRs (e.g. merges/main-to-main-vs-deps)
-        if (IsGitHubReleaseFlowCommit().Match(commit.MessageShort).Success)
+        if (IsGitHubReleaseFlowCommit().Match(commit.MessageShort).Success ||
+            IsGitHubActionCodeFlowCommit().Match(commit.MessageShort).Success)
         {
             mergePRFound = true;
             return true;
@@ -193,6 +194,8 @@ protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage
 
     [GeneratedRegex(@"^Merge pull request #\d+ from dotnet/merges/")]
     private static partial Regex IsGitHubReleaseFlowCommit();
+    [GeneratedRegex(@"^\[automated\] Merge branch '.*' => '.*' \(#\d+\)")]
+    private static partial Regex IsGitHubActionCodeFlowCommit();
     [GeneratedRegex(@"^Merge pull request #(\d+) from")]
     private static partial Regex IsGitHubMergePRCommit();
     [GeneratedRegex(@"\(#(\d+)\)(?:\n|$)")]