Fix warnings about invalid toolsets

Add our custom android_ndk/emsdk toolsets to "valid_tools" for the gmake2 action. These errors were confusing even for people familiar with our build system.

Diffs=
973ff2276 Fix warnings about invalid toolsets (#7300)

Co-authored-by: Chris Dalton <99840794+csmartdalton@users.noreply.github.com>

.rive_head

diff --git a/.rive_head b/.rive_head
index a329f6d..1563e51 100644
--- a/.rive_head
+++ b/.rive_head
@@ -1 +1 @@
-265c009852e86492deb69277ad18aa2c394c93e5
+973ff2276f928634e964fafb2d74f11a1cb9d29c

build/rive_build_config.lua

diff --git a/build/rive_build_config.lua b/build/rive_build_config.lua
index 973f748..f6ff510 100644
--- a/build/rive_build_config.lua
+++ b/build/rive_build_config.lua
@@ -295,6 +295,8 @@
         return android_ndk_tools[tool]
     end
 
+    valid_cc_tools = premake.action._list['gmake2'].valid_tools['cc']
+    valid_cc_tools[#valid_cc_tools + 1] = 'android_ndk'
     toolset('android_ndk')
 
     buildoptions({
@@ -444,6 +446,9 @@
     end
 
     system('emscripten')
+
+    valid_cc_tools = premake.action._list['gmake2'].valid_tools['cc']
+    valid_cc_tools[#valid_cc_tools + 1] = 'emsdk'
     toolset('emsdk')
 
     linkoptions({ '-sALLOW_MEMORY_GROWTH=1', '-sDYNAMIC_EXECUTION=0' })
@@ -461,7 +466,7 @@
     filter({ 'options:arch=wasm', 'options:no-wasm-simd' })
     do
         linkoptions({ '-s MIN_SAFARI_VERSION=120000' })
-    end    
+    end
 
     filter({ 'options:arch=wasm', 'options:config=debug' })
     do