During a package build we call prepare_buildenv in multiple stages of
the process. For debug packages, one of the hooks is buildenv_debugflags
which populates the debug flags to the according variables.
The issue is that the behavior of the current implementation of
buildenv_debugflags is not idempotent, so consecutive calls will append
the same flags again. In certain cases this isn't an issue, however
for context aware build frontends like cargo any change of the build
inputs leads to a fresh build. This means that any invocation of such
a build ecosystem inside the package() function will trigger a full
rebuild, which is not desired.
To fix this issue, this commit makes buildenv_debugflags idempotent
by only appending flags once to the target variables.
Signed-off-by: Levente Polyak <anthraxx@archlinux.org>
