build: --credential-file-override is no longer required (#783)

@chingor13 has configured our runtime environment, such that we no longer need to explicitly provide a credentials file.

How does this magic work, I didn't fully understand?

Source-Author: Benjamin E. Coe <bencoe@google.com>
Source-Date: Fri Aug 7 19:02:09 2020 -0700
Source-Repo: googleapis/synthtool
Source-Sha: 94421c47802f56a44c320257b2b4c190dc7d6b68
Source-Link: googleapis/synthtool@94421c4

Author: yoshi-automation

handwritten/bigtable/.kokoro/populate-secrets.sh

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/nodejs-bigtable.git",
-        "sha": "bb514a9422b727f81595249ec69af63039eb1df6"
+        "sha": "3ca5de3af142d0015209ee1281a767f6fd5a5093"
       }
     },
     {
@@ -19,7 +19,7 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "5936421202fb53ed4641bcb824017dd393a3dbcc"
+        "sha": "94421c47802f56a44c320257b2b4c190dc7d6b68"
       }
     }
   ],