feat: use debug logging when in debug mode (#128) (#129)

* feat: use debug logging when in debug mode (#128)

* chore: remove unnecessary getExperimental function

* chore: fix setupMise typing

As `core.getInput` returns always a string, the type of `version` in
`setupMise` should be just a `string`. When the string is empty it means
to use the latest version. This could be made cleaner but this is how it
works.

* feat: only set MISE_LOG_LEVEL when required

Previously MISE_LOG_LEVEL was always set, even when log_level parameter
wasn't configured. This changes that behaviour so that the environment
variable is only set when log_level is set by the user and that the
MISE_LOG_LEVEL is set to debug only for this action when in debug mode.

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Co-authored-by: Jeff Dickey <[email protected]>