CI build updates

[mauzey1]

Resolves #919

This PR makes the following changes:

• Replaces conda-build with rattler-build, which makes the build step run about 7 times faster.
• Changes the script for rebuilding the recipe YAML file to accommodate cmor-feedstock's rattler-build changes.
• The testing step will install the hdf5plugin into its mamba environment and setup the HDF5_PLUGIN_PATH variable. This is done for change in libnetcdf 4.10.0 that require hdf5plugin for the ZSTD filter. The package udunits2 is also installed for configuring the test Makefile. Udunits2 is no longer a dependency of CMOR but it is needed by the configure script.