Toolchain update: avoid Javascript interpreting git log (#3421)

git log entries may themselves use backticks, which prematurely ended
the string (and then caused Javascript syntax errors). Avoid this
problem by using the environment variable rather than having Javascript
see the string contents (of that environment variable).

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

Co-authored-by: Adrian Palacios <73246657+adpaco-aws@users.noreply.github.com>

.github/workflows/toolchain-upgrade.yml

@@ -64,7 +64,7 @@ jobs:
  https://github.com/rust-lang/rust/commit/${{ env.next_toolchain_hash }}. The log
  for this commit range is:
 
- ${{ env.git_log }}`
+ ` + process.env.git_log
  })
 
  - name: Create Issue