Fix macOS Ice installation detection

[externl]

ice-builder-gradle/src/main/groovy/com/zeroc/gradle/icebuilder/slice/SliceExtension.groovy

Line 133 in cffcc02

return "/usr/local"

We currently assume that Ice will be installed to /usr/local. With our default installation using brew on Apple Silicon, this will typically be /opt/homebrew/.

We should probably check a few places and/or check for brew --prefix.