config: Update Darwin config to include a directory relative to $BINDIR

Look for helpers such as `gvproxy` under `$BINDIR/../libexec/podman`, which
helps this work on distributions such as Homebrew without the need to hardcode
paths into the binary, which makes a pre-built binary distribution work
regardless of the installation prefix.

Tested: Updated vendored package in podman, built it with `podman-remote`,
copied `gvproxy` to a `libexec/podman` at the same level as `bin/podman` and
confirmed that `podman machine start` worked as expected.

Signed-off-by: Filipe Brandenburger <filbranden@gmail.com>

pkg/config/config_darwin.go

@@ -35,4 +35,6 @@ var defaultHelperBinariesDir = []string{
 	"/usr/local/lib/podman",
 	"/usr/libexec/podman",
 	"/usr/lib/podman",
+	// Relative to the binary directory
+	"$BINDIR/../libexec/podman",
 }