[Tests] set $_ before sourcing nvm.sh in fast tests

nvm.sh uses `NVM_SCRIPT_SOURCE="$_"` to detect its source location.
Adding `: nvm.sh` before each source line ensures `$_` is set correctly, preventing breakage when the previous command (e.g., `set -ex`) overwrites it.

test/fast/Unit tests/nvm_get_arch_unofficial

@@ -15,6 +15,7 @@ die() {
   exit 1
 }
 
+: nvm.sh
 . ../../../nvm.sh
 
 # Sets the PATH for these tests to include the symlinks to the mocked binaries