Refactor staging step for download strategies.

[reitermarkus]

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Refactor the staging step to explicitly include the destination directory in all commands.

Extracted from #4402.

[MikeMcQuaid]

Thanks for splitting this out, dramatically easier to review. Looks good 👍

[ilovezfs]

Command output seems to be significantly uglier.
Before

==> Verifying gdb-8.1.tar.xz checksum
tar xJf /Users/joe/Library/Caches/Homebrew/gdb-8.1.tar.xz

After

==> Verifying gdb-8.1.tar.xz checksum
tar -x -J -f /Users/joe/Library/Caches/Homebrew/gdb-8.1.tar.xz -C /private/tmp/gdb-20180705-21040-1vk0rfk

I don't think specifying the output directory should have compromised aesthetics.

Also, it calls attention to the useless -J which doesn't actually flag any errors. You can pass -J to a gz archive or -z to an xz archive and tar will just do whatever it wants anyway. Maybe this isn't true on very old versions of macOS but now we're displaying the useless options with a neon sign.

[reitermarkus]

I don't think specifying the output directory should have compromised aesthetics.

I can't change anything about the -C /private/tmp/gdb-20180705-21040-1vk0rfk part,
and xJf vs. -x -J -f is debatable.

[ilovezfs]

When all that's actually needed is xf, I think xJf was barely passable, and -x -J -f is just ugly.

[MikeMcQuaid]

When all that's actually needed is xf, I think xJf was barely passable, and -x -J -f is just ugly.

Agreed here. Let's just xf

[MikeMcQuaid]

Agreed here. Let's just xf

Done in #4431