Don't try to sign git commits when running tests

I have git set up to always sign with a key that requires a password, this would
require me to enter that password when running tests.
Git allows overwriting config for a single invocation, lets just do that

test/rbs/collection/sources/git_test.rb

@@ -58,14 +58,14 @@ def test_resolved_revision_updated_after_fetch
       git "config", "user.name", "Your Name", chdir: origin_repo
       git "checkout", "-b", "main", chdir: origin_repo
 
-      git "commit", "--allow-empty", "-m", "Initial commit", chdir: origin_repo
+      git "-c", "commit.gpgsign=false", "commit", "--allow-empty", "-m", "Initial commit", chdir: origin_repo
       sha_initial_commit = git("rev-parse", "HEAD", chdir: origin_repo).chomp
 
       RBS::Collection::Sources::Git.new(name: "test", revision: "main", remote: origin_repo, repo_dir: "gems").tap do |source|
         assert_equal sha_initial_commit, source.resolved_revision
       end
 
-      git "commit", "--allow-empty", "-m", "Second commit", chdir: origin_repo
+      git "-c", "commit.gpgsign=false", "commit", "--allow-empty", "-m", "Second commit", chdir: origin_repo
       sha_second_commit = git("rev-parse", "HEAD", chdir: origin_repo).chomp
 
       RBS::Collection::Sources::Git.new(name: "test", revision: "main", remote: origin_repo, repo_dir: "gems").tap do |source|