WIP: NixOS compatibility via a secondary graphics stack provider

From @luigipiucco, via https://gitlab.com/luigipiucco/steam-runtime-tools/-/tree/nixos as of a00fe9f8

• pressure-vessel: Search relative to provider

  Some searches were done with hard-coded paths, or paths that didn't take a provider other than / into consideration.

  Also, there may be unusual cases in which libc is not a link, but a regular file. This accommodates for that.

• runtime: Add some paths

• system-info: Unset bad variables when sysroot != /

  This makes loader and XDG variables be ignored if the provider is not plain root, assuming that, if someone created a sysroot just for pressure-vessel (such as in NixOS), they used the default paths.