mrustc-bootstrap: Document workarounds

pkgs/development/compilers/mrustc/bootstrap.nix

@@ -72,7 +72,7 @@ stdenv.mkDerivation rec {
   makeFlags = [
     # Use shared mrustc/minicargo/llvm instead of rebuilding them
     "MRUSTC=${mrustc}/bin/mrustc"
-    #"MINICARGO=${mrustc-minicargo}/bin/minicargo"  # FIXME: uncomment this so minicargo doesn't get rebuilt
+    #"MINICARGO=${mrustc-minicargo}/bin/minicargo"  # FIXME: we need to rebuild minicargo locally so --manifest-overrides is applied
     "LLVM_CONFIG=${llvm_12.dev}/bin/llvm-config"
     "RUSTC_TARGET=${rust.toRustTarget stdenv.targetPlatform}"
   ];

pkgs/development/compilers/mrustc/patches/0001-dont-download-rustc.patch

@@ -1,3 +1,6 @@
+The $(RUSTC_SRC_DL) file already exists, but for some reason Make wants to rebuild
+this target when it has $(RUSTC_SRC_TARBALL) as a dependency.
+
 --- a/minicargo.mk	2023-04-06 08:26:18.408817572 +0200
 +++ b/minicargo.mk	2023-04-06 08:27:11.553536996 +0200
 @@ -176,7 +176,7 @@