fix Nushell tests

because this was developed when Nushell was in 0.95.0, some tweaks had to be added

• allow to set the Nushell binary with the NU var in Makefile, e.g. make NU="nu-0.95.0" test if you have Nushell 0.95.0 installed as nu-0.95.0 in PATH
• use $nu.current-exe for nested Nushell invocation to preserve the version set in the top-level Makefile
• use --no-config-file to make sure there is no interaction with the user's config, which might not be for the correct version

Makefile

+NU="nu"
 .PHONY: fmt fmt-check check clippy test show show-proxy unset-proxy toto
 
 ssh_addr_file = 0
@@ -19,9 +20,9 @@ clippy:
 test:
 	cargo build --release
 ifeq ($(ssh_addr_file),0)
-	nu help_func/execute_all_tests.nu
+	${NU} help_func/execute_all_tests.nu
 else
-	nu help_func/execute_all_tests.nu --ssh-addr-file $(ssh_addr_file)
+	${NU} help_func/execute_all_tests.nu --ssh-addr-file $(ssh_addr_file)
 endif
 
 show:
@@ -30,7 +31,7 @@ show:
 	rustc --version
 	cargo --version
 	cargo clippy --version
-	nu --version
+	${NU} --version
 
 show-proxy:
 	@echo "HTTP_PROXY = ${HTTP_PROXY}"