Toolchain update: avoid Javascript interpreting git log

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).

.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