modules/lua: fix the build

The assignment to PREFIX env var had no effect on the makefile
variable 'PREFIX', so it resolved to an enpty string. We need to put
it inside eval.

This fixes the build for me:

$ make image=cli
...
make[2]: Entering directory '/home/tgrabiec/src/osv/modules/lua'
PREFIX=/home/tgrabiec/src/osv/modules/lua
cd build/luarocks-2.1.2 && \
    ./configure --prefix=/out --force-config \
    --with-lua-include=/src --lua-version=5.2
Lua interpreter found: /usr/bin/lua5.2...
Lua version detected: 5.2
Looking for Lua... lua5.2 found in $PATH: /usr/bin
Checking Lua includes... lua.h not found (looked in /src, /src/lua/5.2, /src/lua5.2)
You may want to use the flag --with-lua or --with-lua-include. See --help.

configure failed.

Makefile:75: recipe for target 'build/luarocks-2.1.2/Makefile' failed
make[2]: *** [build/luarocks-2.1.2/Makefile] Error 1
make[2]: Leaving directory '/home/tgrabiec/src/osv/modules/lua'

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>