install: Fix IS_MAC checks

install

@@ -68,7 +68,7 @@ for bin in $BASE/bin/*; do
   fi
 done
 
-if [ $IS_MAC ] ; then
+if [ $IS_MAC == 1 ] ; then
   # Homebrew
   [ -z "$(which brew)" ] &&
     ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
@@ -96,7 +96,7 @@ fi
 
 # Add and install these ones.
 ADD_AND_INSTALL="golang ripgrep ag fd bat jq nodejs fzf github-cli"
-if [ $IS_MAC ] ; then
+if [ $IS_MAC == 1 ] ; then
   echo "Detected macOS, installing extra packages via asdf."
   ADD_AND_INSTALL="$ADD_AND_INSTALL git tmux vim"
 fi
@@ -118,7 +118,7 @@ for plugin in $ADD_ONLY ; do
 done
 unset ADD_ONLY
 
-if [ $IS_MAC ] ; then
+if [ $IS_MAC == 1 ] ; then
   echo 'To install SBCL:'
   echo 'CPATH=$(brew --prefix)/include:$CPATH LIBRARY_PATH=$(brew --prefix)/lib:$LIBRARY_PATH asdf install sbcl latest'
 fi