Lowercase git url for rust-lang/enzyme.git

On Fuchsia, we have an internal Gerrit mirrors of the rust repositories
to avoid excess load on the public github servers. Since rust uses
submodules, we need to then use git's `url.<base>.insteadOf` to point
our checkouts at our mirrors.

We'd prefer to be able to point all repositories under
`https://github.com/rust-lang` to
`https://rust.googlesource.com/rust-lang`, but unfortunately it seems
that when Rust mirrored Enzyme, the repository name was lower cased to
`https://github.com/rust-lang/enzyme`, but kept the name capitalized in
the .gitmodules file. This didn't cause a problem for Github, which
seems to handle repository names in a case insensitive way, Gerrit is
case sensitive, so we can't use a glob rule. Instead we have to setup
`insteadOf` rules for each repository.

This renames the URL to match the case of the repository name, which
should avoid the issue.

Author: erickt

.gitmodules

@@ -45,7 +45,7 @@
 	shallow = true
 [submodule "src/tools/enzyme"]
 	path = src/tools/enzyme
-	url = https://github.com/rust-lang/Enzyme.git
+	url = https://github.com/rust-lang/enzyme.git
 	shallow = true
 [submodule "src/gcc"]
 	path = src/gcc