Inherit env and disable noop piping for emulators:exec.

[yuchenshi]

Description

This change makes the script of firestore emulators:exec inherit the environment variables (e.g. PATH). This should make sure firestore emulators:exec "npm test" always uses the local npm as expected.

It also disables piping of stdout and stderr since we're not changing them in anyway. As a nice side benefit, most CLI tools will now detect stdout as a terminal and turn on nice coloring, etc.

Scenarios Tested

Manually: firestore emulators:exec "env" to make sure environment variables are inherited. (As opposed to before where it only has the firestore emulator env var.) Also, manually firestore emulators:exec "which node" now correctly shows my local nvm node.

Manually firestore emulators:exec "npm test" in quickstart-nodejs/firestore-emulator/typescript-quickstart to make sure mocha prints nice colored output, as opposed to no colors before.

[samtstern]

@yuchenshi thanks! Approved but don't forget the changelog.

[coveralls]

Coverage remained the same at 61.446% when pulling f42ba6b on ys/emulators-exec-env-pipe into 6e9066f on master.

[coveralls]

Coverage remained the same at 61.446% when pulling 139836e on ys/emulators-exec-env-pipe into 6e9066f on master.