stdenv/setup.sh: make sure $sourceRoot has +x before cd-ing

This change is prompted by the following, admittedly cursed tarball:

```
> curl https://registry.npmjs.org/char-regex/-/char-regex-1.0.2.tgz 2>/dev/null \
  | tar -ztv
drw-rw-rw- 0/0               0 2020-02-18 10:50 package
-rw-rw-rw- 0/0             297 2020-02-18 10:50 package/index.d.ts
-rw-rw-rw- 0/0            1920 2020-02-18 10:50 package/index.js
-rw-rw-rw- 0/0            1092 2020-01-31 11:31 package/LICENSE
-rw-rw-rw- 0/0             937 2020-02-18 10:51 package/package.json
-rw-rw-rw- 0/0             713 2020-02-18 10:50 package/README.md
```

The minimal reproducer for the issue is the following derivation trying
to work around the uid 0 issue with `dontMakeSourcesWritable = true`:

```nix
{ stdenv, fetchurl }:

stdenv.mkDerivation {
  name = "test";

  src = fetchurl {
    sha1 = "d744358226217f981ed58f479b1d6bcc29545dcf";
    url = "https://registry.npmjs.org/char-regex/-/char-regex-1.0.2.tgz";
  };

  dontMakeSourcesWritable = true;

  installPhase = ''
    cp -R . $out
  '';
}
```

This currently fails in the following way:

```
these derivations will be built:
  /nix/store/pc3jbydl0xcc8nrndf5xkf7hdhpgpb41-test.drv
building '/nix/store/pc3jbydl0xcc8nrndf5xkf7hdhpgpb41-test.drv'...
unpacking sources
unpacking source archive /nix/store/v9p98kqplf4kflmy91p0687xlvr6klb1-char-regex-1.0.2.tgz
source root is package
find: 'package/index.d.ts': Permission denied
find: 'package/index.js': Permission denied
find: 'package/LICENSE': Permission denied
find: 'package/package.json': Permission denied
find: 'package/README.md': Permission denied
/nix/store/6c47azxacncswc1pllzj28zfzqw40d7c-stdenv-linux/setup: line 1311: cd: package: Permission denied
builder for '/nix/store/pc3jbydl0xcc8nrndf5xkf7hdhpgpb41-test.drv' failed with exit code 1
error: build of '/nix/store/pc3jbydl0xcc8nrndf5xkf7hdhpgpb41-test.drv' failed
```

As you can see, the issue is that `$sourceRoot` isn't executable,
prohibiting the call to `cd`. This can be fixed by running
`chmod +x "${sourceRoot}"` before `cd` regardless of
`dontMakeSourcesWritable` in `unpackPhase` since if `chmod` fails, `cd`
would fail as well and we are out of options.

Verified that the workaround works locally.

Another thing to investigate is investigating if we should use
`--no-same-owner` for `tar` and if it helps in this case as well.
See also <https://github.com/Profpatsch/yarn2nix/issues/56>.

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