treewide: use 'lib.getExe' when possible

pkgs/ff2mpv-go/default.nix

@@ -12,7 +12,7 @@ buildGoModule rec {
   vendorHash = null;
 
   postPatch = ''
-    sed -i -e 's,"mpv","${mpv}/bin/mpv",' ff2mpv.go
+    sed -i -e 's,"mpv","${lib.getExe mpv}",' ff2mpv.go
   '';
 
   postInstall = ''

pkgs/unbound-zones-adblock/default.nix

@@ -10,7 +10,7 @@ stdenvNoCC.mkDerivation {
   installPhase =
     let
       gawkCmd = lib.concatStringsSep " " [
-        ''${gawk}/bin/awk''
+        (lib.getExe gawk)
         '''{sub(/\r$/,"")}''
         ''{sub(/^127\.0\.0\.1/,"0.0.0.0")}''
         ''BEGIN { OFS = "" }''