ci: try to install NetSNMP from Homebrew

ci: try to install NetSNMP from Homebrew

The one shipped with MacOS is broken.

```
checking whether C compiler supports flag "-DNETSNMP_ENABLE_IPV6 -fno-strict-aliasing -g -Os -pipe -DBUILD=174 -DMACOSX_DEPLOYMENT_TARGET=12.6 --sysroot /Applications/Xcode_14.0.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.3.sdk -iframeworkwithsysroot /System/Library/Frameworks/-1788-11ed-b9c4-96898e02b808/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.6.Internal.sdk/System/Library/PrivateFrameworks/ -1788-11ed-b9c4-96898e02b808/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.6.Internal.sdk/System/Library/Frameworks/ -iwithsysroot /usr/local/libressl/include -L/AppleInternal/Library/BuildRoots/a0876c02-1788-11ed-b9c4-96898e02b808/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.6.Internal.sdk/usr/local/libressl/lib -Udarwin21 -Ddarwin21=darwin21 -I/usr/local/include -DNETSNMP_NO_INLINE -lnetsnmpmibs -lnetsnmpagent -lnetsnmp"
```