glib-compile-schemas is part of dev output of glib package. Installation will not include dev output by default.
For environment.systemPackages, you can have it included in the buildEnv result using environment.extraOutputsToInstall = [ "dev" ]; option. Or just explicitly install glib.dev.
But I would not recommend that. Globally installing libraries is considered a bad practice. The Nix way is to use nix-shell -p glib, which will include dev output for glib (since nix-shell essentially puts the packages into buildInputs).
And do not even try nix-env, unless you find pain convenient.