fix([email protected]): support pure = "on"

This switches the `purego` build tag to use the `cgo` tag which
allows the source files to be correctly conditionally included
in the build actions.

There is a second patch that adds a couple of unit tests
that have `pure = "on"` enabled to prevent regressions.

modules/circl/metadata.json

@@ -9,7 +9,8 @@
   "repository": [],
   "versions": [
     "1.3.3",
-    "1.3.7"
+    "1.3.7",
+    "1.3.7.bcr.1"
   ],
   "yanked_versions": {}
 }