refactor(certs): remove redundant branch in file-write logic

[hyperfinitism]

During the review of PR #124 unnecessary conditional branching was discovered in src/certs.rs. This pull request removes that branch to simplify the write path while preserving existing behaviour.

Testing

Environment

• VM: Google Cloud Platform N2D (Milan)
• OS: Ubuntu 24.04.1
• Kernel: 6.14.0-1017-gcp

Steps

# 1. Normal download succeeds
snpguest fetch ca pem . milan
# → ark.pem and ask.pem are created

# 2. Make ark.pem read-only
chmod -w ark.pem

# 3. Attempt to overwrite; expect a write-error
snpguest fetch ca pem . milan

Output

$ chmod -w ark.pem
$ snpguest fetch ca pem . milan
ERROR: unable to create or overwrite ark certificate
because: Permission denied (os error 13)
Error: unable to create or overwrite ark certificate

Caused by:
    Permission denied (os error 13