internal/pkg: optional executable files

This enables a FileArtifact implementation to specify whether its resulting file should be executable. This change also implements shebang and ELF magic checks in the file artifact.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/compress.go

@@ -91,6 +91,9 @@ func (c compoundCloser) Close() error {
 	return err
 }
 
+// IsExecutable returns false.
+func (*decompressArtifact) IsExecutable() bool { return false }
+
 // Cure returns a decompressor [io.ReadCloser].
 func (a *decompressArtifact) Cure(r *RContext) (io.ReadCloser, error) {
 	sr, err := r.Open(a.f)