bootc install to-existing-root failed on aarch64 test

The failure comes from bootc commit 0a757685ee631ca9d53df6941af4fb71e4ac1bf3 or 0a757685ee631ca9d53df6941af4fb71e4ac1bf3, because commit 51dabaa5cb7b6313b8cd75a002862c64de3c7b85 passed.

efibootmgr needs write access to /sys/firmware/efi/efivars to modify EFI boot variables (like deleting boot entries with -B)
This PR is a simple fix, mount /sys as read-write.

```
podman run --rm --tls-verify=false --privileged --pid=host -v /:/target -v /dev:/dev -v /var/lib/containers:/var/lib/containers --security-opt label=type:unconfined_t localhost/bootc:tmt bootc install to-existing-root --target-transport containers-storage
Installing image: ostree-unverified-image:containers-storage:localhost/bootc:tmt
Digest: sha256:c8de8390f6dcf4a9f6477d29fef5c04ff2ab8faa32ebb61d942f16ea8c0cce34
----------------------------
WARNING: This operation will OVERWRITE THE BOOTED HOST ROOT FILESYSTEM and is NOT REVERSIBLE.
Waiting 20s to continue; interrupt (Control-C) to cancel.
----------------------------
Initializing ostree layout
layers already present: 0; layers needed: 73 (2.2 GB)
Deploying container image...done (33 seconds)
Bootloader: grub
Installing bootloader via bootupd
Executing: "efibootmgr" "-b" "0002" "-B"
Could not delete variable: Read-only file system
error: boot data installation failed: installing component EFI: Updating EFI firmware variables: Clearing EFI boot entries that match target Red Hat Enterprise Linux: Failed to run command: Command {
program: "efibootmgr",
args: [
"efibootmgr",
"-b",
"0002",
"-B",
],
create_pidfd: false,
}
error: Installing to filesystem: Installing bootloader: Failed to run command: Command {
program: "bwrap",
args: [
"bwrap",
"--bind",
"/target/ostree/deploy/default/deploy/96b094a8419f0fd81c0434730e7906d00a3571017a8755761aab66df4c7270ea.0",
"/",
"--proc",
"/proc",
"--dev",
"/dev",
"--ro-bind",
"/sys",
"/sys",
"--bind",
"/target/boot",
"/boot",
"--dev-bind",
"/dev/nvme0n1",
"/dev/nvme0n1",
"--dev-bind",
"/dev/nvme0n1p1",
"/dev/nvme0n1p1",
"--dev-bind",
"/dev/nvme0n1p2",
"/dev/nvme0n1p2",
"--dev-bind",
"/dev/nvme0n1p3",
"/dev/nvme0n1p3",
"--setenv",
"PATH",
"/bin:/usr/bin:/sbin:/usr/sbin:/usr/local/bin:/usr/local/sbin",
"--",
"bootupctl",
"backend",
"install",
"--write-uuid",
"--update-firmware",
"--auto",
"--device",
"/dev/nvme0n1",
"/",
],
create_pidfd: false,
}
```