[build][gardening] obtain CMAKE_OSX_SYSROOT value once (#29098)

Supports rdar://problem/58347344

Author: edymtt

utils/build-script-impl

@@ -537,15 +537,17 @@ function set_build_options_for_host() {
                 done
             fi
 
+            cmake_os_sysroot="$(xcrun --sdk ${platform} --show-sdk-path)"
+
             cmark_cmake_options=(
                 -DCMAKE_C_FLAGS="$(cmark_c_flags ${host})"
                 -DCMAKE_CXX_FLAGS="$(cmark_c_flags ${host})"
-                -DCMAKE_OSX_SYSROOT:PATH="$(xcrun --sdk ${platform} --show-sdk-path)"
+                -DCMAKE_OSX_SYSROOT:PATH="${cmake_os_sysroot}"
                 -DCMAKE_OSX_DEPLOYMENT_TARGET="${cmake_osx_deployment_target}"
             )
             llvm_cmake_options=(
                 -DCMAKE_OSX_DEPLOYMENT_TARGET:STRING="${cmake_osx_deployment_target}"
-                -DCMAKE_OSX_SYSROOT:PATH="$(xcrun --sdk ${platform} --show-sdk-path)"
+                -DCMAKE_OSX_SYSROOT:PATH="${cmake_os_sysroot}"
                 -DCOMPILER_RT_ENABLE_IOS:BOOL=FALSE
                 -DCOMPILER_RT_ENABLE_WATCHOS:BOOL=FALSE
                 -DCOMPILER_RT_ENABLE_TVOS:BOOL=FALSE