test: fstab-generator: adjust PATH for fsck

test: fstab-generator: adjust PATH for fsck

fsck(8) is located in /usr/sib/ on Debian sid:

    stdout:
    *** Running /home/christian/Coding/workspaces/systemd/test/testdata/test-fstab-generator/test-01-dev-nfs.input
    *** Running /home/christian/Coding/workspaces/systemd/test/testdata/test-fstab-generator/test-02-dhcp.input
    *** Running /home/christian/Coding/workspaces/systemd/test/testdata/test-fstab-generator/test-03-dhcp6.input
    *** Running /home/christian/Coding/workspaces/systemd/test/testdata/test-fstab-generator/test-04-nfs.input
    *** Running /home/christian/Coding/workspaces/systemd/test/testdata/test-fstab-generator/test-05-nfs4.input
    *** Running /home/christian/Coding/workspaces/systemd/test/testdata/test-fstab-generator/test-06-ipv4.input
    *** Running /home/christian/Coding/workspaces/systemd/test/testdata/test-fstab-generator/test-07-ipv6.input
    *** Running /home/christian/Coding/workspaces/systemd/test/testdata/test-fstab-generator/test-08-implicit-nfs.input
    *** Running /home/christian/Coding/workspaces/systemd/test/testdata/test-fstab-generator/test-09-cifs.input
    *** Running /home/christian/Coding/workspaces/systemd/test/testdata/test-fstab-generator/test-10-iscsi.input
    *** Running /home/christian/Coding/workspaces/systemd/test/testdata/test-fstab-generator/test-11-live.input
    *** Running /home/christian/Coding/workspaces/systemd/test/testdata/test-fstab-generator/test-12-dev-sdx.input
    --- /dev/fd/63  2022-11-04 15:39:13.131532174 +0100
    +++ /dev/fd/62  2022-11-04 15:39:13.131532174 +0100
    @@ -6,3 +6,4 @@
     initrd-usr-fs.target.requires
     initrd-usr-fs.target.requires/sysroot.mount
     sysroot.mount
    +systemd-fsck-root.service
    **** Unexpected output for /home/christian/Coding/workspaces/systemd/test/testdata/test-fstab-generator/test-12-dev-sdx.input
    stderr:
    Skipping root directory handling, as root on NFS was requested.
    Skipping root directory handling, as root on NFS was requested.
    Skipping root directory handling, as root on NFS was requested.
    Skipping root directory handling, as root on NFS was requested.
    Skipping root directory handling, as root on NFS was requested.
    Skipping root directory handling, as root on NFS was requested.
    Skipping root directory handling, as root on NFS was requested.
    Skipping root directory handling, as root on NFS was requested.
    Skipping root directory handling, as root on CIFS was requested.
    Skipping root directory handling, as root on iSCSI was requested.
    Skipping root directory handling, as root on live image was requested.
    Found entry what=/dev/sdx1 where=/sysroot type=n/a opts=ro
    Checking was requested for /dev/sdx1, but the fsck command does not exist.