Ignore runner token prefix in short_sha

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR fixes a behavior introduced in this milestone in Implement support for runner token prefix (!110275 - merged) for Add support for optional runner authentication ... (#383198 - closed). In that MR we introduced a token prefix, but given that short_sha is computed from the first 9 characters of the token, we need to ignore the prefix if it is present.

NOTE: I haven't added a changelog trailer since the functionality is not released yet.

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[16] pry(main)> Ci::Runner.find(10001).short_sha
  Ci::Runner Load (3.1ms)  SELECT "ci_runners".* FROM "ci_runners" WHERE "ci_runners"."id" = 10001 LIMIT 1
=> "glrt-t9W"
[17] pry(main)> reload! # load the code from this MR
[18] pry(main)> Ci::Runner.find(10001).short_sha
  Ci::Runner Load (1.2ms)  SELECT "ci_runners".* FROM "ci_runners" WHERE "ci_runners"."id" = 10001 LIMIT 1 
=> "t9Wkyj-HG" # the prefix is no longer returned in the short sha

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