cargoSetupHook: fix setting crt-static

As is retrospectively obvious from the fact that it was looking at
pkgsTargetTarget, this should have been in the section for the target,
not the host.  This fixes e.g. pkgsStatic.xq for 64-bit RISC-V.

(For platforms other than 64-bit RISC-V this being wrong was masked by
the musl Rust targets being static by default.)

pkgs/build-support/rust/hooks/default.nix

@@ -109,13 +109,13 @@
           lib.optionalString (stdenv.hostPlatform.config != stdenv.targetPlatform.config) ''
             [target."${stdenv.targetPlatform.rust.rustcTarget}"]
             "linker" = "${pkgsTargetTarget.stdenv.cc}/bin/${pkgsTargetTarget.stdenv.cc.targetPrefix}cc"
+            "rustflags" = [ "-C", "target-feature=${
+              if pkgsTargetTarget.stdenv.targetPlatform.isStatic then "+" else "-"
+            }crt-static" ]
           ''
           + ''
             [target."${stdenv.hostPlatform.rust.rustcTarget}"]
             "linker" = "${stdenv.cc}/bin/${stdenv.cc.targetPrefix}cc"
-            "rustflags" = [ "-C", "target-feature=${
-              if pkgsTargetTarget.stdenv.targetPlatform.isStatic then "+" else "-"
-            }crt-static" ]
           '';
       };
       passthru.tests =