rubocops/shell_commands: add cop for shell metacharacters in exec

[Bo98]

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Invoking the shell in exec causes the process to be detached from brew, which causes problems when running pkill in formula tests.

[BrewTestBot]

Review period will end on 2021-05-04 at 19:27:57 UTC.

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

Great work here @Bo98!