Provide env var access to --fsiargs build script args so works on FAKE 3

[bentayloruk]

This is my proposal to fix #502 so --fsiargs is workable again in FAKE 3. See #502 for more details.

[forki]

what's the difference to normal build script parameters?

[bentayloruk]

Sorry, I wasn't notified of your comment! I just randomly came to have a look.

tl;dr - the --fsiargs switch previously merged, is broken in FAKE 3. This is the fix.

The longer story...

My previous merged PR with the CLI changes and --fsiargs #455, was the result of my desire to be able to access the specific set of arguments passed to the build script, from within my build script. These and only these arguments. The reason for this, is that I have build scripts that use UnionArgParser, and UAP will not ignore unrecognised arguments.

At the time, all build script args were put in to environment variables (this is still supported). I thought of changing UAP to access these, but it would still not work as there could be user and machine level vars in the dictionary too (hence the set was not explicitly limited). Therefore, I added support for them to passed to FSI explicitly (with the --fsiargs switch). However, --fsiargs was broken in FAKE 3 (as I can see no way of splitting the FSI options and actual build script args when setting up the FsiEvaluationSession), so I need some other way to access just the build script args. Hence this PR!