cache_store: handle corrupt DBM database.

[MikeMcQuaid]

When the DBM database cannot be read by the current version of Ruby's DBM library (due to corruption or another incompatibility) it segfaults or freezes which takes down the entire Homebrew Ruby process.

This isn't desirable so instead perform a shell out with the Homebrew Ruby to see if it can read the DBM database before we try to use the information. If this hangs or crashes: silently delete the database and recreate it.

Fixes #4923.

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[claui]

(Heads up: I will be away from my computer until tomorrow.)

[MikeMcQuaid]

@claui No worries, I'll merge this as-is as it's somewhat urgent and welcome any post-merge review.