devtools: fix test of ninja install

devtools: fix test of ninja install

When trying to compile some examples with libdpdk.pc,
the right environment (for default target) was not loaded.
The consequence is to not detect some dependencies because
of missing directories in PKG_CONFIG_PATH.

The environment preparation is moved to a dedicate function,
and called for the default target (cc),
before testing the install output of the default build.

Fixes: 272236741258 ("devtools: load target-specific compilation environment")

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>