C:\Users\a\.cargo\bin>rustup default stable
info: syncing channel updates for 'stable-x86_64-pc-windows-msvc'

C:\Users\a\.cargo\bin>

C:\Users\a\.cargo\bin>rustup toolchain install stable
info: syncing channel updates for 'stable-x86_64-pc-windows-msvc'

C:\Users\a\.cargo\bin>

C:\Users\a\.cargo\bin>rustup --verbose update stable
verbose: read metadata version: '12'
verbose: installing toolchain 'stable-x86_64-pc-windows-msvc'
verbose: toolchain directory: 'C:\Users\a\.rustup\toolchains\stable-x86_64-pc-windows-msvc'
info: syncing channel updates for 'stable-x86_64-pc-windows-msvc'
verbose: creating temp file: C:\Users\astra\.rustup\tmp\cizkbcsni3xp7r1c_file
verbose: downloading file from: 'https://static.rust-lang.org/dist/channel-rust-stable.toml.sha256'
verbose: downloading with reqwest

C:\Users\a\.cargo\bin>

OK that's interesting, it implies something is wrong with how reqwest is looking for stuff on your system and thus it fails
(a) it shouldn't silently fail
and (b) eventually RUSTUP_USE_CURL is going away