test stdenv derivation: set dontInstall

In a "normal" Rust crate, the installPhase will do nothing, since stdenv
setup.sh will look for a file called Makefile, and then bail out when it
doesn't find it.

However, in a Rust crate that has a file called Makefile, the
installPhase will run, and will do broken things. In particular, I see
consistent failures of the form

```
...
++ [[ -z '' ]]
++ [[ -e Makefile ]]
++ foundMakefile=1
++ '[' -n /nix/store/1avx3g6yx60xgbfvrzalysz572cr8ghg-run-tests-rust_secp256k1-zkp-0.11.0 ']'
++ mkdir -p /nix/store/1avx3g6yx60xgbfvrzalysz572cr8ghg-run-tests-rust_secp256k1-zkp-0.11.0
mkdir: cannot create directory '/nix/store/1avx3g6yx60xgbfvrzalysz572cr8ghg-run-tests-rust_secp256k1-zkp-0.11.0': File exists
+ exitHandler
+ exitCode=1
...
```

This derivation was introduced in nix-community#357; previously we simply used
runCommand which has no notion of "phases". When we switched to stdenv
we moved the runCommand functionality into buildPhase and didn't
consider that any of the other default phases might do bad stuff.

crate2nix/templates/nix/crate2nix/default.nix

@@ -154,6 +154,8 @@ rec {
               ${testCommand}
             done
           '';
+
+           dontInstall = true;
         };
     in
     pkgs.runCommand "${crate.name}-linked"