Merge bitcoin/bitcoin#31276: guix: scope pkg-config to Linux only

bcd82b13f4 Remove pkgconfig from toolchain file (TheCharlatan)
319a4e8261 depends: drop sqlite pkgconfig file (fanquake)
a8fe1fd38b depends: better cleanup after fontconfig (fanquake)
17e79c9260 depends: fully remove libtool archives from Qt build (fanquake)
8ca85651c8 guix: move pkg-config to Linux builds (fanquake)
e3e648cf41 depends: drop pkg-config option from Qt build (fanquake)
0d185bd99f doc: update depends doc to prefer .cmake outputs (fanquake)

Pull request description:

  After #31181, `pkg-config` is no-longer needed for macOS or Windows Guix builds. It's still needed for Linux, as it's used by a Qt subdependency (fontconfig to find freetype). However we should also no-longer need it for Qt itself, when building using depends.

ACKs for top commit:
  TheCharlatan:
    ACK bcd82b13f4

Tree-SHA512: 89ae68281030d43fcb6c5c96429cd038a21f13a8ca19ea828ada47e8f9f0aa7407854a67c9003652817e47ab9565573b7028342e3e11bb1cca1d823c483081cd

contrib/guix/manifest.scm

@@ -511,7 +511,6 @@ inspecting signatures in Mach-O binaries.")
         gcc-toolchain-12
         cmake-minimal
         gnu-make
-        pkg-config
         ;; Scripting
         python-minimal ;; (3.10)
         ;; Git
@@ -527,6 +526,7 @@ 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")