package/pkg-meson.mk: fix ccache auto-detection avoidance

package/pkg-meson.mk: fix ccache auto-detection avoidance

Commit f4a61d1ae23e (package/pkg-meson.mk avoid host ccache detection)
forced the host C and C++ compilers so that meson does not try to
autodetect ccache, and instead relies on what we provide.

However, this incorrectly used single-expansion of variables in a
package infra.

For traditional builds, this is OK, because the value does not change
across packages.

However, for builds with per-package directories, this value only refers
to the generic path, which ill not exist until the end of the build when
all packages are aggregated in the host-finalize step.

Fix that by postponing the variable evaluation like all the others.

Reported-by: Xogium on IRC
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>