Enable support for ubuntu-derived linux-es in installer
Checks /etc/os-release for mention of ubuntu and debian.
Linux Mint uses ID_LIKE=ubuntu, not ID_LIKE=debian (as seen in
Ubuntu 16.04, unsure about others)
* Pin avr-gcc in shell.nix pending release of 8.3.0
There's apparently a critical bug in 8.2.0, which is now the nixpkgs default. This change overrides that default in favor of the known good version. Once 8.3.0 is the default, the override can be dropped.
* Arch/Manjaro fix
