tests-stdenv-gcc-stageCompare: delay asserts

The removed assert is covered by `meta.platforms = linux`, which will
throw an "unsupported system" error on darwin. Since the platforms are
restricted to linux anyway, it doesn't make sense to have this set to
`all` in `release-small.nix` - `linux` should suffice. Right?

The other assert is delayed until after the evaluation of `meta`. This
allows the "unsupported system" error on darwin to be thrown at all,
without triggering the assert before. This helps CI, which can't catch
asserts properly, but can do so with `meta.platforms` based errors.

(cherry picked from commit 3ea96163cc)

pkgs/test/stdenv/gcc-stageCompare.nix

@@ -14,8 +14,6 @@
   lib,
 }:
 
-assert stdenv.cc.isGNU;
-
 with pkgs;
 # rebuild gcc using the "final" stdenv
 let
@@ -31,11 +29,23 @@ let
         NIX_OUTPATH_USED_AS_RANDOM_SEED = stdenv.cc.cc.out;
       });
 in
-assert lib.assertMsg (gcc-stageCompare ? checksum)
-  "tests-stdenv-gcc-stageCompare: No `checksum` output in `gcc-stageCompare` see conditional in `gcc/common/checksum.nix`";
-(runCommand "gcc-stageCompare" { } ''
-  diff -sr ${pkgs.gcc-unwrapped.checksum}/checksums ${gcc-stageCompare.checksum}/checksums && touch $out
-'').overrideAttrs
+
+(runCommand "gcc-stageCompare"
+  {
+    checksumCompare =
+      assert lib.assertMsg (gcc-stageCompare ? checksum)
+        "tests-stdenv-gcc-stageCompare: No `checksum` output in `gcc-stageCompare` see conditional in `gcc/common/checksum.nix`";
+      gcc-stageCompare.checksum;
+
+    checksumUnwrapped =
+      assert lib.assertMsg (pkgs.gcc-unwrapped ? checksum)
+        "tests-stdenv-gcc-stageCompare: No `checksum` output in `gcc-stageCompare` see conditional in `gcc/common/checksum.nix`";
+      pkgs.gcc-unwrapped.checksum;
+  }
+  ''
+    diff -sr "$checksumUnwrapped"/checksums "$checksumCompare"/checksums && touch $out
+  ''
+).overrideAttrs
   (a: {
     meta = (a.meta or { }) // {
       platforms = lib.platforms.linux;

pkgs/top-level/release-small.nix

@@ -175,5 +175,5 @@ in
   xfsprogs = linux;
   xkeyboard_config = linux;
   zip = all;
-  tests-stdenv-gcc-stageCompare = all;
+  tests-stdenv-gcc-stageCompare = linux;
 }))