Fix mask_secrets in shell_command to Use Mutable String

What does this MR do and why?

We try to perform a gsub operation on a frozen string in mask_secrets in shell_command.rb, causing this error: gitlab-org/gitlab#374553 (closed)

This changes input to return a duplicated, mutable string if it is frozen, or the string itself, if not frozen.

This operator was also used in the previous mask_secrets method before gitlab-org/gitlab-qa!1031 (merged) was implemented: https://gitlab.com/gitlab-org/gitlab-qa/-/blob/43a3c403457f4d93de4a1ce50cb316d09db00b7a/lib/gitlab/qa/docker/command.rb#L52

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