ci: explicitly checkout develop branch of modflow6, simplify steps

[wpbonelli]

• bump actions/checkout to v3
• use actions/checkout option ref: develop
  • the action's default behavior is to checkout the default branch — this makes sure MODFLOW 6 repo's develop branch is used for the nightly build even if the default branch changes (e.g., to master)
• use pwsh for ifort build on Windows (makes system path adjustment for MSVC unnecessary)

[wpbonelli]

This PR previously pinned the flopy dependency to avoid breaking the build if issues make it past that repo's CI, but this change has been removed following discussion with @langevin-usgs and @jdhughes-usgs — given the tight coupling between modflow6 and flopy, we want the nightly build to expose integration issues at the bleeding edge.