cargo_common.bbclass: Add missing space in shell conditional code

Fixes cofigure errors in rust recipes e.g.

tmp/work/riscv32-yoe-linux-musl/rust/1.62.1-r0/temp/run.setup_cargo_environment.3047487: line 215: [: missing `]'

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>