fix: ignore EBADF on lgetxattr

[lionello]

Relates to #2201, but it's not exactly the same issue, since this PR addresses the error during reading of the extended attributes, not the writing:

INFO[2024-08-15T17:28:26Z] Taking snapshot of full filesystem...        
error building image: error building stage: failed to take snapshot: failed to read "security.capability" attribute from "/etc/alternatives/c++": lgetxattr security.capability /etc/alternatives/c++: bad file descriptor

This happened on DigitalOcean App Platform, where Kaniko was running with minimal capabilities (and this cannot be relaxed.)

Output of capsh --print

Current: cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap=ep
Bounding set =cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap
Ambient set = <unsupported>
Current IAB: !cap_dac_read_search,!cap_linux_immutable,!cap_net_broadcast,!cap_net_admin,!cap_net_raw,!cap_ipc_lock,!cap_ipc_owner,!cap_sys_module,!cap_sys_rawio,!cap_sys_ptrace,!cap_sys_pacct,!cap_sys_admin,!cap_sys_boot,!cap_sys_nice,!cap_sys_resource,!cap_sys_time,!cap_sys_tty_config,!cap_lease,!cap_audit_control,!cap_mac_override,!cap_mac_admin,!cap_syslog,!cap_wake_alarm,!cap_block_suspend,!cap_audit_read,!cap_perfmon,!cap_bpf,!cap_checkpoint_restore
Securebits: 037777777777/0xffffffff/32'b11111111111111111111111111111111 (no-new-privs=1)
 secure-noroot: yes (locked)
 secure-no-suid-fixup: yes (locked)
 secure-keep-caps: yes (locked)
uid=0(root) euid=0(root)
gid=0(root)
groups=0(root)
Guessed mode: PURE1E_INIT (2)

Description

This PR checks EBADF errno when the call to lgetxattr fails. This seems to fail in some environments with missing capabilities. Ignoring the error (and skipping the xattr) is better than aborting or ignoring the file completely (which is the workaround posted in #2201)

This is not easy to test in unit/integration tests, but I've verified the fix works with a project that kept failing with the error mentioned in #2201: trying run Kaniko in a Digital Ocean App building this project: https://github.com/DefangLabs/samples/tree/main/samples/flask

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Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

- ignore "bad file descriptor" (EBADF) while trying to read xattrs

[lionello]

@hypnoce @imjasonh can one of you have a look please? It's been open for quite a while now.