Update install.sh to use PYBIN

open_spiel/scripts/install.sh

@@ -25,6 +25,11 @@ die() {
 set -e  # exit when any command fails
 set -x  # show evaluation trace
 
+PYBIN=${PYBIN:-"python${OS_PYTHON_VERSION}"}
+PYBIN=${PYBIN:-"python"}
+PYBIN=${PYBIN:-"python3"}
+PYBIN=`which $PYBIN`
+
 MYDIR="$(dirname "$(realpath "$0")")"
 
 # Calling this file from the project root is not allowed,
@@ -279,8 +284,8 @@ elif [[ "$OSTYPE" == "darwin"* ]]; then  # Mac OSX
     brew list [email protected] && brew unlink [email protected]
     brew link --force --overwrite "python@${OS_PYTHON_VERSION}"
   fi
-  `python -c "import tkinter" > /dev/null 2>&1` || brew install tcl-tk || echo "** Warning: failed 'brew install tcl-tk' -- continuing"
-  python --version
+  `${PYBIN} -c "import tkinter" > /dev/null 2>&1` || brew install tcl-tk || echo "** Warning: failed 'brew install tcl-tk' -- continuing"
+  ${PYBIN} --version
   [[ -x `which clang++` ]] || die "Clang not found. Please install or upgrade XCode and run the command-line developer tools"
   [[ -x `which curl` ]] || brew install curl || echo "** Warning: failed 'brew install curl' -- continuing"
   if [[ ${OPEN_SPIEL_BUILD_WITH_GO:-"OFF"} == "ON" ]]; then
@@ -294,8 +299,8 @@ elif [[ "$OSTYPE" == "darwin"* ]]; then  # Mac OSX
   fi
 
   curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
-  python get-pip.py
-  python -m pip install virtualenv
+  ${PYBIN} get-pip.py
+  ${PYBIN} -m pip install virtualenv
 else
   echo "The OS '$OSTYPE' is not supported (Only Linux and MacOS is). " \
        "Feel free to contribute the install for a new OS."