Merge bitcoin/bitcoin#24340: util: Add missing unlinkat to syscall sandbox

fa45597 util: Add missing unlinkat to syscall sandbox (MarcoFalke)

Pull request description:

  This will be needed for g++-12 (after libstdc++6 12-20220206).

  Steps to reproduce:

  ```
  gdb --args ./src/bitcoind -sandbox=log-and-abort -regtest
  ./src/bitcoin-cli -regtest -named createwallet wallet_name=a descriptors=false
  ./src/bitcoin-cli -regtest stop
  ```

  BT:

  ```
  Thread 1 "b-shutoff" received signal SIGSYS, Bad system call.
  0x00007ffff79564f7 in unlinkat () from /lib/x86_64-linux-gnu/libc.so.6
  (gdb) bt
  #0  0x00007ffff79564f7 in unlinkat () from /lib/x86_64-linux-gnu/libc.so.6
  #1  0x00007ffff7cc7335 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
  #2  0x00007ffff7cc94e3 in std::filesystem::remove_all(std::filesystem::__cxx11::path const&) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
  #3  0x00005555559d4918 in wallet::BerkeleyEnvironment::Flush (this=0x7fffc4005160, fShutdown=<optimized out>) at /usr/include/c++/12/bits/fs_path.h:595
  #4  0x000055555592c058 in wallet::StopWallets (context=...) at /usr/include/c++/12/bits/shared_ptr_base.h:1665
  #5  0x00005555556617ca in Shutdown (node=...) at ./src/init.cpp:293
  #6  0x000055555563ada6 in AppInit (argv=<optimized out>, argc=<optimized out>, node=...) at ./src/bitcoind.cpp:249
  #7  main (argc=<optimized out>, argv=<optimized out>) at ./src/bitcoind.cpp:273

ACKs for top commit:
  laanwj:
    Code review ACK fa45597

Tree-SHA512: e80a38828f8656040954c9befa2d1c9d5170e204dc09c61031633349897f51ccd85cc5c99a089c4726d7f5237875cd9ed3fa8ef864cd6c1c8a2b8250b392d57f

Author: MarcoFalke

upgrade-tracking/commit-database.csv

@@ -497,3 +497,4 @@ fa6065661a86656a29e89ed1a3529cb7103f5394,2022-02-11,MarcoFalke,"refactor: Avoid
 876b91c3833d388279baeae82e3efcb3d462f2ca,2022-02-13,Jeremy Rand,"release-process: Specify remote name in "git fetch"",success,none,automatic,upgrade-tracking/patches/success/876b91c3833d388279baeae82e3efcb3d462f2ca.patch,Clean cherry-pick
 b1aa3b1959cab1ad8bb6d0a6a5d05c87c1271cdb,2022-02-13,MarcoFalke,"Merge bitcoin/bitcoin#24330: doc: release-process: Specify remote name in "git fetch"",success,modified,patch-modifier,upgrade-tracking/patches/modified/b1aa3b1959cab1ad8bb6d0a6a5d05c87c1271cdb.patch,Applied with DigiByte preservation
 6b8e4977b8323b35856ea9f6892cda98c8a2ba58,2022-02-15,MarcoFalke,"Merge bitcoin/bitcoin#24342: test: remove `import socket` in test_ipv6_local",success,modified,patch-modifier,upgrade-tracking/patches/modified/6b8e4977b8323b35856ea9f6892cda98c8a2ba58.patch,Applied with DigiByte preservation
+493b1156d84377428450632340bfde887ca7a25a,2022-02-15,fanquake,"Merge bitcoin/bitcoin#24345: doc: Release process: fix broken link to Guix building docs",success,modified,patch-modifier,upgrade-tracking/patches/modified/493b1156d84377428450632340bfde887ca7a25a.patch,Applied with DigiByte preservation

upgrade-tracking/last-processed.txt

@@ -1 +1 @@
-6b8e4977b8323b35856ea9f6892cda98c8a2ba58
+493b1156d84377428450632340bfde887ca7a25a