Avoid use of git:// URLs in submodules

[RyanGlScott]

GitHub recently removed support for git:// URLs. Unfortunately, this causes problems for SAW, at least one submodule makes use of a git:// URL. See this failing CI job for an example:

Submodule 'blst' (https://github.com/supranational/blst) registered for path 'blst'
Submodule 'cryptol-specs' (git://github.com/GaloisInc/cryptol-specs) registered for path 'cryptol-specs'
Cloning into '/workdir/blst'...
Cloning into '/workdir/cryptol-specs'...
fatal: remote error: 
  The unauthenticated git protocol on port 9418 is no longer supported.
Please see https://github.blog/2021-09-01-improving-git-protocol-security-github/ for more information.
fatal: clone of 'git://github.com/GaloisInc/cryptol-specs' into submodule path '/workdir/cryptol-specs' failed

This arises from this line in the blst-verification repo. It's possible that there are others, too, since heapster.saw resorts to this trick:

saw-script/heapster.dockerfile

Lines 37 to 39 in 09463dd

	# Necessary to deal with SSH URLs in some submodules
	RUN git config --global url."https://github.com/".insteadOf git@github.com: && \
	git config --global url."https://".insteadOf git://

Now that git:// is officially no longer supported, we should:

1. Patch all affected submodules upstream to remove their use of git:// URLs,
2. Update the commits in saw-script's Dockerfiles accordingly, and
3. Remove any uses of git config --global url."https://".insteadOf git:// in the Dockerfiles to ensure that everything works as expected.