Load Archive::Tar only when used (in Dist::Builder, Plugin::TestRelease)

This patch replaces `use Archive::Tar` with appropriate `require Archive::Tar` in `Dist::Zilla::Dist::Builder` and `Dist::Zilla::Plugin::TestRelease`.
One less module loaded every time.

Thanks to [Devel::TraceUse](https://metacpan.org/module/Devel::TraceUse) for the help:
```
perl -d:TraceUse -Ilib -MDist::Zilla -e '$Dist::Zilla::VERSION=$Dist::Zilla::Plugin::GatherDir::VERSION=4.300030; require "bin/dzil"' nop
```