One last fix for tests failing on non amd64:
----------------------------------------------------------------------
FAIL: makebootable_test.go:791: makeBootable20Suite.TestMakeRunnableSystem20RunModeSealKeyErr
makebootable_test.go:893:
c.Assert(err, IsNil)
... value *errors.errorString = &errors.errorString{s:"cannot copy trusted asset to cache: read /tmp/check-855608922/49/run/mnt/ubuntu-seed: is a directory"} ("cannot copy trusted asset to cache: read /tmp/check-855608922/49/run/mnt/ubuntu-seed: is a directory")
Signed-off-by: Maciej Borzecki <maciej.zenon.borzecki@canonical.com>