Fix out-of-tree builds

Since b16df9ee out-of-tree builds have been broken because we
started validating the newly user-configurable $erratatxt file
before linking it into the build dir. We fix this not by moving
the validation, but by removing the linking and instead using the
full path of the $erratatxt file. This allows one to keep that file
separate from the src and build dirs.

Fixes: b16df9ee ("arch-run: Add reserved variables to the default environ")
Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20200511070641.23492-1-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>