Subtree Update Automation Workflow Bugs

[carolynzech]

#248 creates a pull request to update subtree/library. There are a few issues I discovered while using it:

1. The last commit it introduces is erroneous; it should not be creating new submodules. (See how it creates a new library/ folder inside the existing library folder). In CI, the Kani workflow tries to initialize these submodules, but the .gitmodules file doesn't mention them, so CI crashes.

The fix for this issue should produce the exact same output as #248 minus this one erroneous commit.

2. verify-rust-std/.github/workflows/update-subtree.yml

   Lines 122 to 124 in b764eeb

   	sed -i "s/^channel = \"nightly-.*\"/channel = \"${NEXT_TOOLCHAIN_DATE}\"/" rust-toolchain.toml
   	git -c user.name=gitbot -c user.email=git@bot \
   	commit -m "Update toolchain to ${NEXT_TOOLCHAIN_DATE}" rust-toolchain.toml

should instead be:

sed -i "s/^channel = \"nightly-.*\"/channel = \"nightly-${NEXT_TOOLCHAIN_DATE}\"/" rust-toolchain.toml
git -c user.name=gitbot -c user.email=git@bot \
 commit -m "Update toolchain to ${NEXT_TOOLCHAIN_DATE}" rust-toolchain.toml

so that rust-toolchain.toml reads nightly-{DATE} instead of just {DATE}