refactor: provide usage when payload sha is missing

[joonas]

Description

Prior to this change, zarf-injector will panic if it's not provided a payload as an argument:

$ ./zarf-injector 
thread 'main' panicked at src/main.rs:261:35:
index out of bounds: the len is 1 but the index is 1
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
zsh: abort      ./zarf-injector

With this change, zarf-injector will return usage as expected and exit gracefully:

$ ./zarf-injector 
Usage: ./zarf-injector <sha256sum>

Related Issue

I don't believe there's a related issue for this, but I figured it was worth submitting a fix for it since I came across this.

Checklist before merging

• Test, docs, adr added or updated as needed
• Contributor Guide Steps followed

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

[schristoff]

This is a great PR, thank you so much! I'm going to check that importing std doesn't put us under our 1MiB size limit.