guix: move pkg-config to Linux builds

This is no-longer needed for macOS or Windows, and is only required on
Linux for a Qt sub dependency (fontconfig to find freetype).

contrib/guix/manifest.scm

@@ -512,7 +512,6 @@ inspecting signatures in Mach-O binaries.")
         gcc-toolchain-12
         cmake-minimal
         gnu-make
-        pkg-config
         ;; Scripting
         python-minimal ;; (3.10)
         ;; Git
@@ -528,6 +527,7 @@ inspecting signatures in Mach-O binaries.")
                  osslsigncode))
           ((string-contains target "-linux-")
            (list bison
+                 pkg-config
                  (list gcc-toolchain-12 "static")
                  (make-bitcoin-cross-toolchain target)))
           ((string-contains target "darwin")