Improve teampass cron detection.

[corentin-soriano]

This change allows administrators to disable the exec() function without break teampass (disable_functions in php.ini).

This also avoids SELinux AVC in audit logs:

type=AVC msg=audit(1730880782.169:329): avc:  denied  { read } for  pid=10249 comm="unix_chkpwd" name="shadow" dev="dm-0" ino=16812627 scontext=system_u:system_r:httpd_t:s0 tcontext=system_u:object_r:shadow_t:s0 tclass=file permissive=1
type=AVC msg=audit(1730880782.169:329): avc:  denied  { open } for  pid=10249 comm="unix_chkpwd" path="/etc/shadow" dev="dm-0" ino=16812627 scontext=system_u:system_r:httpd_t:s0 tcontext=system_u:object_r:shadow_t:s0 tclass=file permissive=1
type=AVC msg=audit(1730880782.169:330): avc:  denied  { getattr } for  pid=10249 comm="unix_chkpwd" path="/etc/shadow" dev="dm-0" ino=16812627 scontext=system_u:system_r:httpd_t:s0 tcontext=system_u:object_r:shadow_t:s0 tclass=file permissive=1
type=AVC msg=audit(1730880782.170:331): avc:  denied  { nlmsg_relay } for  pid=10248 comm="crontab" scontext=system_u:system_r:httpd_t:s0 tcontext=system_u:system_r:httpd_t:s0 tclass=netlink_audit_socket permissive=1
type=AVC msg=audit(1730880782.170:331): avc:  denied  { audit_write } for  pid=10248 comm="crontab" capability=29  scontext=system_u:system_r:httpd_t:s0 tcontext=system_u:system_r:httpd_t:s0 tclass=capability permissive=1
type=AVC msg=audit(1730880782.170:334): avc:  denied  { open } for  pid=10248 comm="crontab" path="/var/spool/cron/httpd" dev="dm-0" ino=16971141 scontext=system_u:system_r:httpd_t:s0 tcontext=system_u:object_r:user_cron_spool_t:s0 tclass=file permissive=1