Add init.environ.rc to aosp_riscv64

init.environ.rc has a postinstall command to create the symlinks in
and directories in the root filesystem.  Without those packaging
target-files.zip fails with:
failed to find system in canned fs_config

Bug: 250918230
Test: lunch aosp_riscv64-userdebug && m installclean && m droid dist
Change-Id: Iccb6ba53a3caa918431119230256a01d4919f1cc