Trac #29345: replace a few obsolete "-a" tests with "-e".

The "-a" test in bash means "file exists," but the standard POSIX "-e"
is more portable and works in bash anyway, so let's use that.

src/bin/sage-env

@@ -463,7 +463,7 @@ unset R_PROFILE
 if [ -d "$SAGE_LOCAL/lib/R/share" ] ; then
    R_MAKEVARS_SITE="$SAGE_LOCAL/lib/R/share/Makevars.site" && export R_MAKEVARS_SITE
    if ! [ -f "$R_MAKEVARS_SITE" ] ; then
-       if ! [ -a "$R_MAKEVARS_SITE" ] ; then
+       if ! [ -e "$R_MAKEVARS_SITE" ] ; then
            echo "## Empty site-wide Makevars file for Sage's R" > "$R_MAKEVARS_SITE"
        else
            >&2 echo "Warning: $R_MAKEVARS_SITE exists and is not a file : trouble ahead..."
@@ -472,15 +472,15 @@ if [ -d "$SAGE_LOCAL/lib/R/share" ] ; then
 fi
 if [ -d "$DOT_SAGE" ] ; then
     if ! [ -d "$DOT_SAGE/R" ] ; then
-        if ! [ -a  "$DOT_SAGE/R" ] ; then
+        if ! [ -e  "$DOT_SAGE/R" ] ; then
             mkdir -p "$DOT_SAGE/R"
         else
             >&2 echo "Warning: $DOT_SAGE/R exists and is not a directory : trouble ahead..."
         fi
     fi
     R_MAKEVARS_USER="$DOT_SAGE/R/Makevars.user" && export R_MAKEVARS_USER
     if ! [ -f "$R_MAKEVARS_USER" ] ; then
-        if ! [ -a "$R_MAKEVARS_USER" ] ; then
+        if ! [ -e "$R_MAKEVARS_USER" ] ; then
             echo "## Empty user-specific Makevars file for Sage's R" > "$R_MAKEVARS_USER"
         else
             >&2 echo "Warning: $R_MAKEVARS_USER exists and is not a file : trouble ahead..."