Add MIMA_CHECK_DIRECTION environment input for direction

[lolgab]

This allows people to dynamically check for different directions, for example having multiple Github Actions checks for, one with backward and one with forward to check if source compatibility might be broken.

[lefou]

This is a good idea and I wanted something like this myself. Conceptually this is also ok, as the mimaReportBinaryIssues target is a command, which is not cached.

Please note, that in upcomming Mill 0.11, you don't need a separate input target; you can just make the rhs of mimaCheckDirection an T.input. (com-lihaoyi/mill#2402)

[lolgab]

Great! I will keep in mind your suggestion to avoid the separate input when I will drop Mill 0.10.

[lefou]

This allows people to dynamically check for different directions, for example having multiple Github Actions checks for, one with backward and one with forward to check if source compatibility might be broken.

Just nitpicking, you can't check source compatibility with this, as MiMa operates on byte code.

[lolgab]

@lefou Yes I know, but it's what people are using to have an approximation of it and also to have a way to check if new functionality has been added or not.
As an example, the README of sbt-version-policy says:

[MiMa] can only detect binary incompatibilities. To detect source incompatibilities, this
plugin uses MiMa in forward mode as an approximation. This is not always correct and may
lead to false positives or false negatives. This is a known limitation of the current
implementation.