rust minimum version needs to be bumped

While building on a system that had a slightly older toolchain, I noticed the following build error:

$ gmake setup
cd tools/helios-build && cargo build --quiet
error[E0658]: use of unstable library feature 'is_terminal'
 --> src/common.rs:8:5
  |
8 | use std::io::IsTerminal;
  |     ^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #98070 <https://github.com/rust-lang/rust/issues/98070> for more information

error[E0658]: use of unstable library feature 'is_terminal'
  --> src/common.rs:20:26
   |
20 |     if std::io::stdout().is_terminal() {
   |                          ^^^^^^^^^^^
   |
   = note: see issue #98070 <https://github.com/rust-lang/rust/issues/98070> for more information

For more information about this error, try `rustc --explain E0658`.
error: could not compile `helios-build` due to 2 previous errors

It appears that the minimum version needs to be bumped to something like 1.70 per https://github.com/rust-lang/rust/releases/tag/1.70.0.

Start by running gmake setup and inspect the Rust configuration for tools/helios-build, then review src/common.rs, where the IsTerminal API causes the reported error. Update the declared minimum Rust version to the version required by this API, such as 1.70, and confirm that the setup build succeeds with that toolchain.