Use an older nightly toolchain to fix spurious memleak issues.

[Alexhuszagh]

After the test cleanup, the test result is ok with no memory leaks in the code, but the cleanup incorrectly reports a memory leak. Since no tests are run, this is not an error in lexical:

    Finished `release` profile [optimized] target(s) in 3m 40s
     Running unittests src/lib.rs (target/release/deps/lexical-cfe9a35d657545f8)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s

       Error leaked 56 B in 1 block
        Info stack trace (user code at the bottom)
             at malloc
             at main
     Summary Leaked 56 B total (0 other errors)
error: test failed, to rerun pass `-p lexical --lib`

Caused by:
  process didn't exit successfully: `/home/runner/.cargo/bin/cargo-valgrind /home/runner/work/rust-lexical/rust-lexical/target/release/deps/lexical-cfe9a35d657545f8` (exit status: 127)
note: test exited abnormally; to see the full output pass --nocapture to the harness.