[goldctl] goldctl imgtest add: Rename flag --add-test-key-file to --add-test-keys-file.
[goldctl] goldctl imgtest add: Rename flag --add-test-key-file to --add-test-keys-file.

This flag is not currently used by any Gold clients so it's safe to rename.

The rationale for this change is to make it consistent with the existing --keys-file flag, and with flags for command "goldctl imgtest check" that will be added in a followup CL.

Bug: skia:9527
Change-Id: I70fa52dc8c2385a1471e0e8c29c7e4a758aa40a8
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/275796
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Commit-Queue: Leandro Lovisolo <f558143c2568be8d55575d13f30a1fc5cb8009d5@google.com>

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