Preserve environment when launching overlay_remounter

fexecve does not preserve the environment by default. The environment is
used to pass certain information to second stage init. Specifically
whether force debuggable is set is passed this way.

Test: Run following steps

vendor/google/tools/flashall --wipe --force_debuggable
adb shell getprop ro.adb.secure
adb root && adb remount && adb reboot
adb shell getprop ro.adb.secure

Without this change, the first getprop would return 0 and the second 1
With this change, both return 0

Bug: 397402912
Change-Id: I4722bd0c2960dd695dc096d8c59c259d12c28a65