test(endtoend): Skip process_plugin_sqlc_gen_json (sqlc-dev#4075)

If the sqlc-gen-json binary isn't installed, skip the test instead of
failing.

Author: kyleconroy

internal/endtoend/testdata/process_plugin_sqlc_gen_json/exec.json

@@ -1,3 +1,4 @@
 {
-  "contexts": ["base"]
+  "contexts": ["base"],
+  "process": "sqlc-gen-json"
 }