Fixes #1198

The compute build command was being invoked even when the user passed their own Wasm binary via compute serve --file <PATH/TO/WASM/BINARY>. This is unnecessary because if you're passing --file then you're indicating that you're not using the Fastly CLI to compile your code as you already have a binary to be used from elsewhere (whether that's an unsupported language like Zig or you have a completely separate build step).

I've modified the compute serve command to skip the compute build if --file is set, and I've also removed some older code where (for some reason) when we were running compute build (even though compute serve --file was set) we were passing through a flag to the compute build command to get it to skip validating the expected ./bin/main.wasm output, which isn't needed anymore if we just skip the build process entirely.