Fix `spacetimedb-update` cargo test on Windows platforms

[rekhoff]

Running cargo test -p spacetimedb-update on a Windows platform (either PowerShell or Git Bash) fails with:

cargo test -p spacetimedb-update --bin spacetimedb-update

    Finished `test` profile [optimized + debuginfo] target(s) in 1.29s
     Running unittests src\main.rs (target\debug\deps\spacetimedb_update-f0e23902f0ebae1c.exe)

running 1 test
test cli::self_install::tests::ensure_script_help_is_up_to_date ... FAILED

failures:

---- cli::self_install::tests::ensure_script_help_is_up_to_date stdout ----

thread 'cli::self_install::tests::ensure_script_help_is_up_to_date' panicked at crates\update\src\cli\self_install.rs:118:14:
couldn't find usage function
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    cli::self_install::tests::ensure_script_help_is_up_to_date

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

error: test failed, to rerun pass `-p spacetimedb-update --bin spacetimedb-update`

The code that is failing is near:

SpacetimeDB/crates/update/src/cli/self_install.rs

Line 115 in a02d8ba

let script = std::fs::read_to_string(concat!(env!("CARGO_MANIFEST_DIR"), "/spacetime-install.sh")).unwrap();

Possibly related to Windows not reading the return line characters correctly.