engine: fix context error wrapping in executor (#7608)

When an exec fails due to context canceled, we were dropping the
cancelation error and just returning the exec error. I believe this got
fat-fingered while updating the code from upstream to use fmt.Errorf
instead of github.com/pkg/errors.

It's not clear if this is really meaningful, but noticed it changed
while attempting to debug occurances of "exit code 137" in our CI, which
plausibly could be related to all of this. It's possible there's calling
code that tries to handle cancelation with errors.Is, which this
would have broken. This is purely hypothetical at this point though.

Signed-off-by: Erik Sipsma <[email protected]>

engine/buildkit/executor.go

@@ -414,7 +414,7 @@ func exitError(ctx context.Context, exitCodePath string, err error) error {
 
 		select {
 		case <-ctx.Done():
-			exitErr.Err = fmt.Errorf(exitErr.Error())
+			exitErr.Err = fmt.Errorf("%s: %w", exitErr.Error(), context.Cause(ctx))
 			return exitErr
 		default:
 			return stack.Enable(exitErr)