Reassign secret token when regenerating one
## What does this MR do?
This is an attempt to fix intermittent errors in out test suite.
```text
Failures:
1) Gitlab::Shell memoized secret_token creates and links the secret token file
Failure/Error: expect(File.read(secret_file).chomp).to eq(secret_token)
expected: "690f959e206ab91acc54e1c605c7ff90"
got: "cccb4e8df9360600271e61114d4e6e68"
(compared using ==)
# ./spec/lib/gitlab/backend/shell_spec.rb:47:in `block (3 levels) in <top (required)>'
```
It appears that `spec/lib/gitlab/backend/shell_spec.rb` tries to change the file that stores secret token, but `Gitlab::Shell` memoizes `@secret_token` on class level, so when it was already created by other tests (`spec/requests/api/internal_spec.rb` in this case), memoized token is not reassigned even if it was generated again with `ensure_secret_token!`.
See merge request !6844
2a0cccc13e