buildMozillaMach: Update native python env var

With Firefox 100.0 the following deprecation warning comes up:

> The "MACH_USE_SYSTEM_PYTHON" environment variable is deprecated,
> please unset it or replace it with either
> "MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE=system" or
> "MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE=none"

And since we want to continue using our own python we're going for the
system value when the version is at least 100.0.

pkgs/applications/networking/browsers/firefox/common.nix

@@ -238,13 +238,17 @@ buildStdenv.mkDerivation ({
     # Set consistent remoting name to ensure wmclass matches with desktop file
     export MOZ_APP_REMOTINGNAME="${binaryName}"
 
-    # Use our own python
-    export MACH_USE_SYSTEM_PYTHON=1
-
     # AS=as in the environment causes build failure
     # https://bugzilla.mozilla.org/show_bug.cgi?id=1497286
     unset AS
 
+  '' + lib.optionalString (lib.versionAtLeast version "100.0") ''
+    # Use our own python
+    export MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE=system
+  '' + lib.optionalString (lib.versionOlder version "100.0") ''
+    # Use our own python
+    export MACH_USE_SYSTEM_PYTHON=1
+
   '' + lib.optionalString (lib.versionAtLeast version "95.0") ''
     # RBox WASM Sandboxing
     export WASM_CC=${pkgsCross.wasi32.stdenv.cc}/bin/${pkgsCross.wasi32.stdenv.cc.targetPrefix}cc