Add cancel-stalling-git-operations input

Adds a new input (default true) that sets GIT_HTTP_LOW_SPEED_LIMIT=1000
and GIT_HTTP_LOW_SPEED_TIME=60 on every git invocation. Git's libcurl
HTTP transport aborts a transfer that stays below 1 KB/s for 60s, which
returns a non-zero exit code so the existing max-attempts retry path
can take over instead of the operation hanging indefinitely.

The env vars are inherited by every git child process, including those
spawned by 'git submodule update' and by 'nsc git-checkout', so the
protection covers the full submodule walk without further plumbing.

Workflow-level overrides win: if the user sets
GIT_HTTP_LOW_SPEED_LIMIT / GIT_HTTP_LOW_SPEED_TIME in their workflow
env, those values are kept. Set cancel-stalling-git-operations: false
to disable entirely.

Amp-Thread-ID: https://ampcode.com/threads/T-019df20f-8464-75f8-9e23-2bd6a9af0c5e
Co-authored-by: Amp <amp@ampcode.com>