validate.sh: --jobs should default to nproc

If `nproc` is available and `--jobs` is not given, this will use the
output of `nproc` as the value for `--jobs`.

Otherwise, the old default value of 4 will be used and a warning will be
printed.

The default value of 4 has remained unchanged since it was added in
6a9a101 in 2018; that may have been a
reasonable number of cores then, but my development machine today has 20
cores, so setting the parallelism to 4 leaves a lot of performance on
the table!

validate.sh

@@ -12,7 +12,7 @@
 #   See https://github.com/haskell/cabal/issues/8049
 HC=ghc
 CABAL=cabal
-JOBS=4
+JOBS=""
 LIBTESTS=true
 CLITESTS=true
 CABALSUITETESTS=true
@@ -293,6 +293,15 @@ fi
 # Adjust runtime configuration
 #######################################################################
 
+if [ -z "$JOBS" ]; then
+    if command -v nproc >/dev/null; then
+        JOBS=$(nproc)
+    else
+        echo "Warning: \`nproc\` not found, setting \`--jobs\` to default of 4."
+        JOBS=4
+    fi
+fi
+
 TESTSUITEJOBS="-j$JOBS"
 JOBS="-j$JOBS"