check-all: Don't canonicalize directory name

Instead just cd to the repository root for pyflakes.  This is a marginally
worse user experience, but should work on Mac OS where we don't have readlink
-f.

(imported from commit f3d1bfb086ff05642e438c16094415d98a6dab92)

tools/check-all

@@ -1,13 +1,15 @@
 #!/bin/bash
 
-SCRIPT_DIR="$(readlink -f "$(dirname "$0")")"
+SCRIPT_DIR="$(dirname "$0")"
 EXITCODE=0
 
 if ! "$SCRIPT_DIR"/jslint/check-all; then
     EXITCODE=1;
 fi
 
-if find "$(readlink -f "$SCRIPT_DIR"/..)" -name confirmation -prune -o -name '*.py' -print0 \
+cd "$SCRIPT_DIR"/..;
+
+if find . -name confirmation -prune -o -name '*.py' -print0 \
     | xargs -0 pyflakes \
     | grep -v 'imported but unused'; then