fix: handle cache body file not existing when using etag

[dsherret]

Removes this unwrap:

> ../deno/target/debug/deno install -Afr jsr:@deno/deployctl
Loading: https://jsr.io/@deno/deployctl/meta.json ReloadAll
NOT MODIFIED: https://jsr.io/@deno/deployctl/meta.json

============================================================
Deno has panicked. This is a bug in Deno. Please report this
at https://github.com/denoland/deno/issues/new.
If you can reliably reproduce this panic, include the
reproduction steps and re-run with the RUST_BACKTRACE=1 env
var set and include the backtrace in your report.

Platform: windows x86_64
Version: 1.41.2
Args: ["V:\\deno\\target\\debug\\deno.exe", "install", "-Afr", "jsr:@deno/deployctl"]

thread 'main' panicked at cli\file_fetcher.rs:394:16:
called `Option::unwrap()` on a `None` value
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[lucacasonato]

Fine. But you should stop messing with your local cache 🥲🤣