cargo_common.bbclass: fix bashism
cargo_common.bbclass: fix bashism

While /bin/sh -> dash, var-EXTERNALSRC didn't work

Signed-off-by: Hongxu Jia <d55095dfc8448df446bb9592fdc085838a3794a7@windriver.com>

meta-rust/meta-rust,meta-rust/meta-rust,meta-rust/meta-rust,meta-rust/meta-rust,meta-rust/meta-rust