track down lack of permission reset for logging

[ferringb]

whilst tracing pkgcore#328 (in pkgcore/pkgcore), a sandbox failure was encountered.

beast ~ # pebuild dev-util/diffball clean unpack
executing phase unpack
tee: '/var/log/portage/dev-util/diffball-1.0.1-r2:buildable:20210926-222845.log': Permission denied
tee: '/var/log/portage/dev-util/diffball-1.0.1-r2:buildable:20210926-222845.log': Permission denied
 * ERROR: dev-util/diffball-1.0.1-r2::gentoo failed (unpack phase):
 *   unpack: exitcode 1: nonexistent file: 'diffball-1.0.1.tar.gz'
 * 
 * Call stack:
 *    isolated-functions.bash, line 333:  called __phase_src_unpack
 *                     global, line  62:  called unpack 'diffball-1.0.1.tar.gz' 'diffball-1.0.1-bug_543310_stack_buffer_overflows.patch' 'diffball-1.0.1-bug_708736_cseek_xz_reset_avail_in_out.patch'
 *                 src_unpack, line 145:  called __ebd_ipc_cmd 'unpack' '' 'diffball-1.0.1.tar.gz' 'diffball-1.0.1-bug_543310_stack_buffer_overflows.patch' 'diffball-1.0.1-bug_708736_cseek_xz_reset_avail_in_out.patch'
 *     ebuild-daemon-lib.bash, line  91:  called __ipc_exit '1' 'nonexistent file: 'diffball-1.0.1.tar.gz''
 *     ebuild-daemon-lib.bash, line  75:  called die
 * 
 * If you need support, post the topmost build error, and the call stack if relevant.
 * 
 * Working directory: '/var/tmp/portage/dev-util/diffball-1.0.1-r2/work'
 * --------------------------- ACCESS VIOLATION SUMMARY ---------------------------
 * LOG FILE: "/var/log/sandbox/sandbox-3483877.log"
 * 
VERSION 1.0
FORMAT: F - Function called
FORMAT: S - Access Status
FORMAT: P - Path as passed to function
FORMAT: A - Absolute Path (not canonical)
FORMAT: R - Canonical Path
FORMAT: C - Command Line

F: fopen_wr
S: deny
P: /var/log/portage/dev-util/diffball-1.0.1-r2:buildable:20210926-222845.log
A: /var/log/portage/dev-util/diffball-1.0.1-r2:buildable:20210926-222845.log
R: /var/log/portage/dev-util/diffball-1.0.1-r2:buildable:20210926-222845.log
C: tee -i -a /var/log/portage/dev-util/diffball-1.0.1-r2:buildable:20210926-222845.log 
 * --------------------------------------------------------------------------------

Ignore the unpack complaint, just focus on the logs.

FEATURES="assume-digests binpkg-docompress binpkg-dostrip binpkg-logs buildpkg compressdebug config-protect-if-modified distlocks ebuild-locks fixlafiles ipc-sandbox merge-sync multilib-strict network-sandbox news parallel-fetch pid-sandbox preserve-libs protect-owned qa-unresolved-soname-deps sandbox sfperms splitdebug strict unknown-features-warn unmerge-logs unmerge-orphans userfetch userpriv usersandbox usersync xattr"