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/net.go

@@ -74,6 +74,9 @@ func (e ResponseStatusError) Error() string {
 	return "the requested URL returned non-OK status: " + http.StatusText(int(e))
 }
 
+// IsExecutable returns false.
+func (*httpArtifact) IsExecutable() bool { return false }
+
 // Cure sends the http request and returns the resulting response body reader
 // wrapped to perform checksum validation. It is valid but not encouraged to
 // close the resulting [io.ReadCloser] before it is read to EOF, as that causes