Switch back to Mono 5
Switch back to Mono 5

This partly reverts commit 8cd1dc0c7f7a82dded230770c5cca978eb79f333,
but only the change to `environ` is reverted. The other changes from
8cd1dc0c7f7a82dded230770c5cca978eb79f333 are kept.

sillsdev/LfMerge,sillsdev/LfMerge,sillsdev/LfMerge