…eadLine

When using node `>= 12.17`, `EAGAIN` errors consistently occur when
trying to read from `stdin` when there is no available data. The retry
mechanism for this was to recursively call back `SyncStdio.readLine`,
which could evtnually lead to a "Maximum call stack size exceeded" error
to occur (possibly hidden from the consumer, and later causing a "Stream
closed" error).

This changes how the retry mechanism works so it operates in a `while`
loop instead of making a recursive call, completely avoiding to run into
the growing stack issue.

Fixes aws/aws-cdk#8288
Fixes aws/aws-cdk#5187