Support CARGO_TARGET_DIR environment variable

[thomcc]

Cargo supports letting you specify a single directory for use for all projects via the CARGO_TARGET_DIR environment variable.

See this reddit post for a reason someone might want to do this (I had 170GB in various target files across my machine), although I've heard it can to speed up build times too, since it improves dep cache hits.

I think we would just want to change

rust-android-gradle/plugin/src/main/kotlin/com/nishtahir/CargoBuildTask.kt

Line 31 in fcea238

val targetDirectory = targetDirectory ?: "${module!!}/target"

, but

1. I haven't tested this, and maybe it's a little more complex.
2. It's unclear to me what should happen if both targetDirectory and that variable is specified. I kind of think that the variable should take precedence, as it's what cargo will actually use. But, maybe there's a use case for targetDirectory that I'm missing.