Improve the end-of-build message

[sniku]

xk6 currently makes assumptions that users are familiar with go build system and understand what $PATH is.
New users often assume that xk6 build process will replace their existing k6 installation. This is not the case, instead, the new build is in a $GOPATH directory, which is usually not part of $PATH.

I suggest the following:

1. At the end of the xk6 build process we print instructions on where to locate the new build and how to use it
2. [bonus for extra points] Detect the path of globally installed k6 and give the user the exact command to replace it with the new build (sudo cp /gopath/bin/k6 /usr/local/bin/k6)

Suggestion for the message

***************************************************
* BUILD COMPLETE - FOLLOW THESE STEPS TO CONTINUE *
***************************************************
xk6 has now produced a new k6 binary including "{xk6-redis}" extension. 
The new binary is located in {`/home/sniku/go/bin/k6`}. 
To use it you need to call it directly `./home/sniku/go/bin/k6 run script.js` 
or replace your globally installed k6 with the new one. 

[imiric]

Yeah, that would help clarify things. As well as document it better in the README, it's not very clear what happens by default.

Though I wouldn't suggest replacing the globally installed k6 binary. For one it might conflict with package managers if k6 was installed from a package, but it would also cause confusion if they build a second binary and overwrite a previous one that had another extension. This is really up to the user how they want to handle it (e.g. renaming the binary to k6-redis, etc.), but it should be a conscious decision, not a suggestion from us.

At the end of the day though, xk6 is a tool for developers and advanced users. It expects some background knowledge around setting up their Go environment (which is currently also a common issue, see #36 and on the forum), how $PATH works, and how to run a binary from the current working directory. Until we have a more user friendly way of at least installing extensions, some of which we've discussed, there's not much we can do besides improving the documentation and notices like this.