update installer.sh with improvements from rustup-init.sh

[Gankra]

Includes:

• Better support for illumos' accursed ksh (now pushes through instead of erroring)
• Better support for /proc not being mounted (occurs in docker buildx -- now pushes through instead of erroring)
• Better detection of "oldworld" loongarch (eagerly errors)
• Handles an issue where macos sysctl spews errors
• Better detection of 32-bit userland on aarch64

Notably this does not reverse an earlier decision to drop most of the complex wget/curl ciphersuite detection from rustup-init.sh. Doing so would be a much larger and higher risk change, as we do not use the same hosting infra that rustup uses, and the logic for it is like, 200 lines of evil sh scripting.

We probably do want to eventually reverse that decision, just not today. A casualty of this decision is this update doesn't include the retry logic for downloads they have (because passing retry flags requires a ton of feature-detection logic they include).

Fixes #15

[Gankra]

Here's the curl/wget logic we're not grabbing:

https://github.com/rust-lang/rustup/blob/2cc0829b65f8f879ba9dabd19ac2d1eab616bc82/rustup-init.sh#L622-L866

[Gankra]

Additional context: installer.ps1 doesn't come from rustup, to the best of my knowledge it originates whole cloth from apollo graphql's installers, which don't really get much updating. It also has a lot less corner cases to handle.