Check or unset variable RUSTC_WRAPPER before launching cargo install

[georgik]

If the environment was cleaned by removing all Rust toolchain file and .cargo, but the environment variable RUSTC_WRAPPER remains to be set to the binary which does not exist anymore, the cargo install command will fail:

  Installing cargo-espflash v1.7.0
error: failed to compile `cargo-espflash v1.7.0`, intermediate artifacts can be found at `C:\projects\tmp\AppData\Local\Temp\cargo-installgwG6p0`

Caused by:
  failed to run `rustc` to learn about target-specific information

Caused by:
  could not execute process `C:\Users\User\.cargo\bin\sccache.exe rustc - --crate-name ___ --print=file-names --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=cfg` (never executed)

Caused by:
  The system cannot find the file specified. (os error 2)
Error: command '"cargo" "install" "cargo-espflash"' exited with non-zero status code 101