Merge #15863: scripts and tools: Ensure repos are up-to-date in gitian-build.py

feed98e Ensure repos are up-to-date (Hennadii Stepanov)

Pull request description:

  These steps are provided by the [release process](https://github.com/bitcoin/bitcoin/blob/master/doc/release-process.md#setup-and-perform-gitian-builds).

ACKs for commit feed98:

Tree-SHA512: ad6876d211e524cf6b8dbe4f0f026b77792c8ae3b728e1419f17d5679766603d21c057a7866c183794c814b914a9e4584e16fc501bec77af7e3472a34bd4d913

Author: MarcoFalke

contrib/gitian-build.py

@@ -219,13 +219,20 @@ def main():
     subprocess.check_call(['git', 'checkout', args.commit])
     os.chdir(workdir)
 
+    os.chdir('gitian-builder')
+    subprocess.check_call(['git', 'pull'])
+    os.chdir(workdir)
+
     if args.build:
         build()
 
     if args.sign:
         sign()
 
     if args.verify:
+        os.chdir('gitian.sigs')
+        subprocess.check_call(['git', 'pull'])
+        os.chdir(workdir)
         verify()
 
 if __name__ == '__main__':