ci: split compiletest target-env steps

[Firestar99]

• split compiletest target-env into individual steps for better readability
• add spv1.5 and spv1.6

Future notes:

• when ci: add rust-cache action #526 and this is merged, compiletest-windows will be the longest running job due to the addition of spv1.5 and spv1.6, so we may want to split the windows job in two jobs.