Allow env() stren() to take variable value as input

[victorwon]

Please describe your feature request.
Currently, env() and strenv() use the env VAR name directly to get result, e.g.
myvar=abc yq -n 'env(myvar)'

But it could be even powerful to allow using the value from processing, e.g.
echo "myvar=123" | yq e '.*| strenv(key)'

With the feature suggested it could output 123, but currently it only results in empty output, all because it tries to look up for env VAR(key) instead of env VAR(myvar)