gh-41025: set full path to sage-num-threads.py in sage-env
    
fixes #40889
Apparently we no longer put SAGE_ROOT/src/bin/ in PATH

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URL: #41025
Reported by: Dima Pasechnik
Reviewer(s): Max Alekseyev, user202729

Author: Release Manager

src/bin/sage-env

@@ -580,7 +580,7 @@ case "$SAGE_NUM_THREADS,$SAGE_NUM_THREADS_PARALLEL" in
         # First, figure out the right values for SAGE_NUM_THREADS (default
         # number of threads) and SAGE_NUM_THREADS_PARALLEL (default number of
         # threads when parallel execution is asked explicitly).
-        sage_num_threads_array=$(sage-num-threads.py 2>/dev/null || echo 1 2 1)
+        sage_num_threads_array=$("$(dirname "$0")"/sage-num-threads.py 2>/dev/null || echo 1 2 1)
         sage_num_threads_array="${sage_num_threads_array% *}" # strip third item
         SAGE_NUM_THREADS="${sage_num_threads_array% *}" # keep first item
         SAGE_NUM_THREADS_PARALLEL="${sage_num_threads_array#* }" # keep second item