pkg-config purged from arm-unknown-linux-gnueabihf

[jmagnuson]

pkg-config is getting removed in the apt-get purge, even though it was explicitly installed in common.sh:

https://dev.azure.com/rust-embedded/cross/_build/results?buildId=576&view=logs&j=ed13e0de-1834-5225-9caa-5c6f5a5a1fa1&t=91dd8e19-28bc-5135-e5ed-520472032e91&l=2665

But on armv7-unknown-linux-gnueabihf for instance, this is not happening:

https://dev.azure.com/rust-embedded/cross/_build/results?buildId=576&view=logs&j=36c4a3c9-c17a-548c-f0e4-0d2902437e43&t=26e6497a-1b48-5743-221f-9d1b01950b2c&l=4078

And testing locally:

$ docker run rustembedded/cross:arm-unknown-linux-gnueabihf which pkg-config || echo "not found"
not found

$ docker run rustembedded/cross:armv7-unknown-linux-gnueabihf which pkg-config
/usr/bin/pkg-config

[jmagnuson]

The toolchain installation doesn't apply the same prune filtering that the other dependency shell scripts do--

https://github.com/rust-embedded/cross/blob/8a2f734bb0b422f131b73f1a3804777cb3cf8392/docker/Dockerfile.arm-unknown-linux-gnueabihf#L17

pkg-config depends on xz-utils, both of which end up getting removed.

Any objection to just removing that entry/line? Curl seems useful to have as well.