Fix the printing of the installed nginx version

Previously the nginx version command was failing since the
`nginx-$STACK` binary does not exist, resulting in output like:

```
remote: -----> Installed directory to /app/bin
```

This failure went unnoticed since `pipefail` mode is not enabled.

The nginx binary path has been fixed, and the command now uses
`-v` instead of `-V` since the former only output one line, avoiding
the need to `head -n1`. In addition, the `cut` usage shows more of
the original line in the case of no match.

Fixes #174.

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+* [#177](https://github.com/heroku/heroku-buildpack-static/pull/177) Fix the printing of the installed nginx version
 * [#177](https://github.com/heroku/heroku-buildpack-static/pull/177) Switch to the recommended S3 URL format
 * [#177](https://github.com/heroku/heroku-buildpack-static/pull/177) Remove unused archive caching
 * [#177](https://github.com/heroku/heroku-buildpack-static/pull/177) Fail the build early on unsupported stacks

bin/compile

@@ -22,9 +22,9 @@ esac
 
 mkdir -p $build_dir/bin
 curl -sSf "${nginx_archive_url}" | tar -xzC "${build_dir}/bin"
-nginx_version=$($build_dir/bin/nginx-$STACK -V 2>&1 | head -1 | awk '{ print $NF }')
+nginx_version=$("${build_dir}/bin/nginx" -v |& cut -d '/' -f 2-)
 cp -a $bp_dir/scripts/{boot,config} -t $build_dir/bin/
-echo "-----> Installed ${nginx_version} to /app/bin"
+echo "-----> Installed nginx ${nginx_version} to /app/bin"
 
 mkdir -p $build_dir/config
 cp $bp_dir/scripts/config/templates/mime.types $build_dir/config