proposal: support cross compiling

[myitcv]

In #62, we fixed -run to use runtime.GOOS and runtime.GOARCH.

At the time of writing, cross compiling with gobin does not work. Nor are the cache paths it uses (either globally or module-locally) GOOS/GOARCH-aware.

This issue is a proposal to fully support cross-compiled binaries with gobin.

• we will need to refactor the way we go install in order not to trip over the fact that you cannot go install with GOBIN set when GOOS and GOARCH differ from runtime.GOOS and runtime.GOARCH respectively
• we will need to introduce GOOS and GOARCH into our cache paths

[myitcv]

As part of this we should explicitly say we do not (currently) support build tags, and consequently set GOFLAGS="" in all of our go commands.

[myitcv]

We should add support for -tags and GOFLAGS. The more I think about it, the more I'm convinced that actually the right thing to do is also to install everything to a fully obfuscated path.

[myitcv]

Per @rogpeppe - we could instead only flip to using an obfuscated path in case GOOS, GOARCH, GOFLAGS etc differ from the "defaults".