Add set-env for setting environment variables (#2802)

tests/shell/pipeline/commands/internal.rs

@@ -345,7 +345,7 @@ fn run_custom_command() {
 }
 
 #[test]
-fn set_variables() {
+fn set_variable() {
     let actual = nu!(
         cwd: ".",
         r#"
@@ -358,6 +358,19 @@ fn set_variables() {
     assert_eq!(actual.out, "17");
 }
 
+#[test]
+fn set_env_variable() {
+    let actual = nu!(
+        cwd: ".",
+        r#"
+            set-env TESTENVVAR = "hello world"
+            echo $nu.env.TESTENVVAR
+        "#
+    );
+
+    assert_eq!(actual.out, "hello world");
+}
+
 #[cfg(feature = "which")]
 #[test]
 fn argument_invocation_reports_errors() {