Fix shellcheck warnings

There are a couple of relevant warnings from shellcheck:

> In build.sh line 62:
> apt-get source ${package}
> SC2248: Prefer double quoting even when variables don't contain special characters.
> [...]
> In build.sh line 66:
> tar xf "${package}"_*.debian.tar.xz -C "${package}"-"${version}/"
> SC2140: Word is of the form "A"B"C" (B indicated). Did you mean "ABC" or "A\"B\"C"?
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