graphics/libplacebo: unbreak GLSLANG=on after 1801dc29cec9

compiler.find_library(static: true) ignores LDFLAGS and LIBRARY_PATH
as Meson uses --print-search-dirs instead of trying to link.

src/glsl/meson.build:34:4: ERROR: C++ static library 'SPIRV' not found
src/glsl/meson.build:45:12: ERROR: C++ static library 'MachineIndependent' not found

PR: 270221
Reported by: Philipp Ost
Regressed by: https://code.videolan.org/videolan/libplacebo/-/commit/980f2574aa0f

(cherry picked from commit 506f67ea200f6858dd4f49efb28ccdd33456e0d9)