Fix conflict on heredoc temp files

If two image builds start concurrently and they both use an heredoc with the same EOF marker, then they will use the same filename and overwrite each other.

It's possible for concurrent builds to either use the wrong heredoc content or to fail because the file of one build is deleted by the other, leading to this error message:

```
STEP 13/21: COPY <<EOF /file.conf

Error: building at STEP "COPY <<EOF /file.conf": reading "/var/tmp/EOF": copier: get: globs [/EOF] matched nothing (0 filtered out): no such file or directory
```

Signed-off-by: Kevin Michel <[email protected]>

imagebuildah/stage_executor.go

@@ -451,7 +451,7 @@ func (s *StageExecutor) performCopy(excludes []string, copies ...imagebuilder.Co
 				data = strings.TrimPrefix(data, "\n")
 				// add breakline when heredoc ends for docker compat
 				data = data + "\n"
-				tmpFile, err := os.Create(filepath.Join(parse.GetTempDir(), path.Base(filepath.ToSlash(file.Name))))
+				tmpFile, err := os.CreateTemp(parse.GetTempDir(), path.Base(filepath.ToSlash(file.Name)))
 				if err != nil {
 					return fmt.Errorf("unable to create tmp file for COPY instruction at %q: %w", parse.GetTempDir(), err)
 				}