stdbuf/build: setting `--target-dir` fails build

[jtracey]

The build script for stdbuf unwraps None when the target dir is set to anywhere that doesn't have a "target" or "cargo-install*" dir somewhere between the project root and the target dir in its path.

[jtracey]

(you can work around this in the meantime by nesting one of those names wherever you actually want it)

[Ecordonnier]

I removed this code in 1f2bd34 , so I think this can be closed.