Configuring tests using environment variables

[Ilia-Kosenkov]

Continues discussion in and fixes #30.
The idea is to be able to control how {rextendr} builds rust code in tests using environment variables.
E.g., if tests should be executed using a different toolchain, use REXTENDR_TOOLCHAIN:

Sys.setenv(REXTENDR_TOOLCHAIN = "stable-x86_64-pc-windows-gnu")
rcmdcheck::rcmdcheck(".")

Currently, only two variables are supported, REXTENDR_TOOLCHAIN and REXTENDR_PATCH_CRATES_IO.
Different crates patches can be separated using ';'.

If the environment variable is not set (or it is an equivalent of an empty string), default values from rust_source are used.

[Ilia-Kosenkov]

@clauswilke,
So what is your opinion on this? Should I proceed?

[clauswilke]

Sure, go ahead.

[clauswilke]

For some reason, my local checks fail now with object 'rust_source_defaults' not found. Not sure what is happening.

But also, now that I think about it, as it is currently implemented this is somewhat problematic, as it now requires every additional test to set the toolchain and patch.crates_io arguments. We may have to rethink this.