Use a pip-installed libsupermesh (#3920) #6419

Job	Run time
Set the Docker release tag	3s
Build Firedrake (real, default)	1h 5m 16s
Build Firedrake (complex, --complex, complex)	57m 17s
Build Docker containers / docker_env / Build the firedrake-env container	32m 50s
Build Docker containers / docker_complex / Build the firedrake-complex container	15m 55s
Build Docker containers / docker_vanilla / Build the firedrake-vanilla container	18m 31s
Build Docker containers / docker_docdeps / Build the firedrake-docdeps container	13m 8s
Build Docker containers / docker_firedrake / Build the firedrake container	11m 2s
Build Docker containers / docker_jupyter / Build the firedrake-jupyter container	3m 35s
	3h 37m 37s

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