package-builder: add --bin_root_dir flag

[tstromberg]

This PR allows the Kolide Launcher to be deployed to Fedora SilverBlue, where /usr is read-only. To build for this platform, pass --bin_root_dir=/opt to ./build/package-builder.

Without this flag, Kolide fails to install on Silverblue with:

sudo rpm-ostree install zxkx_thomas-stromberg-0ac07b_kolide-launcher.rpm                                                         
...
Resolving dependencies... done
Relabeling... done
Checking out packages... done
error: Checkout launcher-kolide-k2-1.6.2-1.x86_64: opendir(local): No such file or directory

It turns out that on Silverblue, /usr/local is a symlink to ../var/usrlocal/, which makes rpm-ostree upset with life.

[CLAassistant]

All committers have signed the CLA.

[directionless]

This looks pretty narrow and reasonable, thank you @tstromberg

It's worth noting that making sure autoupdate works may be another ball of wax. But this is clearly a step.

@RebeccaMahany any objections?

[RebeccaMahany]

Seems fine to me!

Setting the update_directory flag to an appropriate location (if /var/kolide-k2/<hostname>/updates isn't suitable) should solve the autoupdate issue as well.