cargo.bbclass: avoid calling which ${RUSTC} with undefined ${RUSTC}

* RUSTC is normally defined in rust.bbclass, but rust-target-config doesn't
  inherit rust.bbclass and RUSTC is then undefined when calling 'which'
  in oe_cargo_build

* causes which help text ending in log.do_compile, e.g.:

| DEBUG: Executing shell function do_compile
| NOTE: Using rust targets from /OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/openssl-sys/0.9.77-r0/rust-targets/
| NOTE: cargo = /OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/openssl-sys/0.9.77-r0/recipe-sysroot-native/usr/bin/cargo
| Usage: which [options] [--] COMMAND [...]
| Write the full path of COMMAND(s) to standard output.
|
|   --version, -[vV] Print version and exit successfully.
|   --help,          Print this help and exit successfully.
|   --skip-dot       Skip directories in PATH that start with a dot.
|   --skip-tilde     Skip directories in PATH that start with a tilde.
|   --show-dot       Don't expand a dot to current directory in output.
|   --show-tilde     Output a tilde for HOME directory for non-root.
|   --tty-only       Stop processing options on the right if not on tty.
|   --all, -a        Print all matches in PATH, not just the first
|   --read-alias, -i Read list of aliases from stdin.
|   --skip-alias     Ignore option --read-alias; don't read stdin.
|   --read-functions Read shell functions from stdin.
|   --skip-functions Ignore option --read-functions; don't read stdin.
|
| Recommended use is to write the output of (alias; declare -f) to standard
| input, so that which can show aliases and shell functions. See which(1) for
| examples.
|
| If the options --read-alias and/or --read-functions are specified then the
| output can be a full alias or function definition, optionally followed by
| the full path of each command used inside of those.
|
| Report bugs to <which-bugs@gnu.org>.
| NOTE: rustc =

(From OE-Core rev: 0068e82f02354729b953e622889f96ca52468d6c)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes-recipe/cargo.bbclass

@@ -49,7 +49,6 @@ oe_cargo_build () {
 	export RUSTFLAGS="${RUSTFLAGS}"
 	bbnote "Using rust targets from ${RUST_TARGET_PATH}"
 	bbnote "cargo = $(which ${CARGO})"
-	bbnote "rustc = $(which ${RUSTC})"
 	bbnote "${CARGO} build ${CARGO_BUILD_FLAGS} $@"
 	"${CARGO}" build ${CARGO_BUILD_FLAGS} "$@"
 }