various ports: simplify cmake prefix path

graphics/openimageio/Portfile

@@ -54,7 +54,7 @@ if {${port_latest}} {
 
     set port_libfmt         libfmt12
     cmake.prefix_path-prepend \
-                            ${prefix}/lib/${port_libfmt}/cmake/fmt
+                            ${prefix}/lib/${port_libfmt}/cmake
     depends_lib-append      port:${port_libfmt}
 } else {
     PortGroup           boost 1.0

net/mtxclient/Portfile

@@ -49,4 +49,4 @@ depends_lib-append  \
 # a new release is available. nheko must also use that version as well.
 cmake.prefix_path-prepend \
                     ${prefix}/lib/libfmt8/cmake \
-                    ${prefix}/libexec/spdlog-fmt8/lib/cmake
+                    ${prefix}/libexec/spdlog-fmt8

net/nheko/Portfile

@@ -60,7 +60,7 @@ patchfiles          MacNotificationDelegate.mm.patch
 # Match libfmt version with spdlog once there's a new release
 cmake.prefix_path-append \
                     ${prefix}/lib/libfmt8/cmake \
-                    ${prefix}/libexec/spdlog-fmt8/lib/cmake
+                    ${prefix}/libexec/spdlog-fmt8
 
 destroot {
     copy ${build.dir}/nheko.app ${destroot}${applications_dir}/Nheko.app