chore!: default to pure mode, rename --pure flag to --impure for switching

docs/cli.md

@@ -4,7 +4,7 @@
 Usage of nixtest:
       --junit string          Path to generate JUNIT report to, leave empty to disable
       --no-color              Disable coloring
-      --pure                  Unset all env vars before running script tests
+      --impure                Don\'t unset all env vars before running script tests
   -s, --skip string           Regular expression to skip tests (e.g., 'test-.*|.*-b')
       --snapshot-dir string   Directory where snapshots are stored (default "./snapshots")
   -f, --tests string          Path to JSON file containing tests (required)

docs/usage.md

@@ -96,10 +96,10 @@ Examples:
     name = "script-test";
     type = "script";
     script = 
-    # there are two modes, "default"/"impure" and "pure"
+    # there are two modes, "default"/"pure" and "impure"
     # in impure mode all env variables etc. from your current session are kept
-    # and are available to the test
-    # to make it more reproducible and cleaner, use --pure to switch to pure
+    # and are available to the test (using --impure).
+    # to make it more reproducible and cleaner, the default is pure
     # mode which will unset all env variables before running the test. That 
     # requires you to set PATH yourself then:
     #