rust minimum version needs to be bumped

[rmustacc]

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.

[rmustacc]

I'm not sure if there's anything other than 1.70 related features as this environment was updated then to 1.76.