Get chromium path from env var on Linux

[deepanjanroy]

For #106

[brendankenny]

I like this change. Do you want to fix it so the test server also runs it this way? Feel free to do a follow up PR instead of adding another commit to this one.

Right now the test server downloads a continuous Chrome build in test/download-chrome.sh, so you would need to

• change chromepath in download-chrome.sh to LIGHTHOUSE_CHROMIUM_PATH and
• drop the manual launch of Chrome on this line in .travis.yml to instead just run your launch-chrome.sh

I think we do need that extra --disable-setuid-sandbox flag for that case (a downloaded continuous build that's not Chrome and we didn't build ourselves), but not when running on Linux in general, which adds another wrinkle. For those that haven't run into this, this has more details: https://chromium.googlesource.com/chromium/src/+/master/docs/linux_suid_sandbox_development.md#Try-bots-and-waterfall

[paullewis]

Happy to merge. Do you want to update the tests as per @brendankenny's suggestion, or roll with this as-is?

[deepanjanroy]

Let me do a separate PR for the tests.

[paullewis]

Done.