revert(os/mkosi): apply rootfs tmpfiles at boot again

[kvinwang]

Reverts #1331.

Applying rootfs.tmpfiles only at build time broke two things:

• mkosi.finalize runs as the invoking user in a rootless build, so systemd-tmpfiles cannot chown the root-owned entries and every rootless make os-image fails (fchownat() of /buildroot/dstack failed, exit 73). CI runs mkosi as root and did not notice.
• /var/lib is a writable overlay at runtime, so the boot-time pass was not a no-op there: it kept /var/lib/tpm2-tss/system/keystore at 0755 root:root. Without it, tpm2-tss-fapi.conf resets the keystore to 2775 tss:tss on every boot.

The boot-time entries that land on the read-only root were harmless no-ops. This replaces #1409 and #1413, which patched each symptom separately.

Verification

• The revert applies cleanly on next; os/mkosi/build.sh lint passes.
• Rootless build and physical TDX boot results will follow in a comment.

[kvinwang]

Physical TDX (tdxlab) validation of next at 635e7a2200 plus this PR, #1414 and the #841 suite (tree ef0f7b94c), without #1409 or #1413:

• Rootless build (mkosi 26, unprivileged user namespaces): the prod, dev and identity-variant images all build. On next alone the finalize step failed with fchownat() … Invalid argument.
• tc-gos-platform-005 passes: at runtime /var/lib/tpm2-tss/system/keystore is 755 and /tapp links to dstack.
• Full sweep, 4 workers, 372 scripted cases: 361 PASS, 7 BLOCKED (NVIDIA CC GPU or Yocto image required), 2 FAIL and 2 ERROR. All four failures were KMS upgrade-matrix cases (tc-kms-upgrade-001/005, tc-int-end-to-end-003, tc-int-failure-se-002) that timed out waiting for the KMS CVM under 4-worker load. All four pass when rerun one at a time on the same tree and images.