CI (Buildkite): for the signed pipeline test, use signature_file instead of signature (#41706)

* CI (Buildkite): for the signed pipeline test, use `signature_file` instead of `signature`

* Base64-decode the signature before writing it to the signature file

Author: DilumAluthge

.buildkite/pipelines/main/0_webui.yml

@@ -21,4 +21,4 @@ steps:
           # verifies the treehash of the pipeline itself and the inputs listed in `inputs`
           signed_pipelines:
             - pipeline: .buildkite/pipelines/main/misc/signed_pipeline_test.yml
-              signature: "U2FsdGVkX18ZrMlORSIi0TvW99eZ3JyUEhntMyXjrOSJ9jAtUohgHc8+dMnWUc6qWjYt8k63wfPrth33SGWDiF814Bm1W5Zk3+R6HgVu6UCrQEI5sLm+SJPbrvET+Jkn"
+              signature_file: .buildkite/pipelines/main/misc/signed_pipeline_test.yml.signature

.buildkite/pipelines/main/misc/signed_pipeline_test.yml.signature

@@ -0,0 +1 @@
+Salted__��NE""�;��יܜ��3%���0-R�`�>t��QΪZ6-�N�����He��_5��[�d��zn�@�@B9���H�ۮ���'