build: allow Nix spa cflags in cgo builds

libspa-0.2.pc ships -fno-strict-aliasing/-fno-strict-overflow which Go's
cgo rejects unless explicitly allowed. This unblocks the PipeWire capture
dependency (go2tv.app/screencast) which uses #cgo pkg-config.

flake.nix

@@ -36,6 +36,10 @@
           PKG_CONFIG_PATH = "${pkgs.libjpeg_turbo.dev}/lib/pkgconfig";
           LD_LIBRARY_PATH = "${pkgs.libjpeg_turbo.out}/lib";
 
+          # Nix's libspa-0.2.pc emits -fno-strict-aliasing/-fno-strict-overflow
+          # which Go's cgo rejects unless explicitly allowed.
+          CGO_CFLAGS_ALLOW = "-fno-strict-overflow|-fno-strict-aliasing";
+
           shellHook = ''
             echo "TeleportFling dev shell ready!"
           '';