Retrieve API key from crowdin.yml

Before, you had to set an environmental variable with the API key for
the generate-i18n-credits.go script. As this API key is already set in
the crowdin.yml file, this change adds code so it is retrieved from
there if the environemntal variable is not set.

Change-Id: I57c8af1c50863efd21e01cfce2745d86db6b8774
diff --git a/tools/i18n/go.mod b/tools/i18n/go.mod
new file mode 100644
index 0000000..f6901ad
--- /dev/null
+++ b/tools/i18n/go.mod
@@ -0,0 +1,5 @@
+module generate-i18n-credits
+
+go 1.16
+
+require gopkg.in/yaml.v2 v2.4.0