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/ir_test.go

@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"io"
 	"io/fs"
-	"math"
 	"reflect"
 	"testing"
 
@@ -154,7 +153,7 @@ func TestRevision(t *testing.T) {
 
 			p := buf.Bytes()
 			p[8] = 0xfd
-			wantErr := pkg.RevisionError{math.MaxUint64 - 2, math.MaxUint64}
+			wantErr := pkg.RevisionError{0xfd, 0}
 			if _, err := c.NewDecoder(
 				bytes.NewReader(p),
 			).Decode(); !reflect.DeepEqual(err, wantErr) {