cargo config: manually specify rustc

since using rust toolchain from rustup, it's probably best to specify
the full path to rustc. the correct one should be called from the main
shell in use (zsh) but when invoked from e.g. bash, another rustc might
get called as PATH is not properly set up there

.cargo/config.toml

@@ -1,3 +1,4 @@
 [build]
 jobs = 2                              # number of parallel jobs, defaults to # of CPUs
+rustc = ".cargo/bin/rustc"            # the rust compiler tool
 rustc-wrapper = ".cargo/bin/sccache"  # enable sccache