fix: Type of rustic-cargo-nextest-exec-command

This commit enhances the `rustic-cargo-nextest-exec-command`
customization option by changing its type from a single string
to a list of strings.

Author: psibi

.gitignore

@@ -5,4 +5,5 @@
 /config.mk
 *autoloads.el
 test/test-project/target
-test/test-project-single-crate/target
+test/test-project-single-crate/target
+.env

rustic-cargo.el

@@ -37,7 +37,7 @@
 
 (defcustom rustic-cargo-nextest-exec-command (list "nextest" "run")
   "Execute command to run nextest."
-  :type 'string
+  :type '(repeat string)
   :group 'rustic-cargo)
 
 (defcustom rustic-cargo-run-exec-command "run"