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.sum b/tools/i18n/go.sum
new file mode 100644
index 0000000..dd0bc19
--- /dev/null
+++ b/tools/i18n/go.sum
@@ -0,0 +1,4 @@
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+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v2 v2.4.0 h1:D8xgwECY7CYvx+Y2n4sBz93Jn9JRvxdiyyo8CTfuKaY=
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