shells/nushell: fix build on armv7

• mark as LTO_UNSAFE on armv6/armv7 to avoid address space exhaustion
• remove custom do-test target to avoid a conflict with cargo.mk's do-test; cargo.mk's default do-test does the job well enough

Approved by: portmgr (build fix blanket)
MFH: 2023Q4

(cherry picked from commit f22649e35840ddc7d3a19b8e9dd643b3f7885e0f)