unlock only possible via file

would it be an idea to be able to encrypt via environment variables?

git-crypt unlock $GITCRYPT_SYMETRIC_KEY

Now that gitlab-ci is able properly secure these, "Secret Variables; These variables will be set to environment by the runner. So you can use them for passwords, secret keys or whatever you want."

Now, I store them as base64 encoded in the var
```
echo $GITCRYPT_SYMETRICKEY_PATH > .git-crypt-base64.key && \
openssl base64 -d -in .git-crypt-base64.key -out .git-crypt.key && \
git-crypt unlock .git-crypt.key \
&& rm .git-crypt*.key
```