ECM-related tests fail after an incremental build

This is because setting the default ECMBIN=ecm is only done when using
the configure test, but not when skipping the configure test because
the spkg is already installed.

Steps to reproduce:
* ecm is not installed in the base os
* Sage make distclean && make succeeeds
* Tests succeed
* re-running ./bootstrap && ./configure && make works
* Tests now fail with PermissionError: [Errno 13] Permission denied: ''

See also:
 * sagemath#37701
 * sagemath#37011 (comment)

build/pkgs/ecm/spkg-configure.m4

@@ -1,6 +1,6 @@
 SAGE_SPKG_CONFIGURE([ecm], [
+    # CHECK - test whether the package is already installed
     m4_pushdef([SAGE_ECM_MINVER],[7.0.4])
-    ECMBIN=ecm
     SAGE_SPKG_DEPCHECK([gmp], [
         AC_CHECK_HEADER(ecm.h, [
             AX_ABSOLUTE_HEADER([ecm.h])
@@ -35,5 +35,12 @@ SAGE_SPKG_CONFIGURE([ecm], [
         ], [sage_spkg_install_ecm=yes])
     ])
     m4_popdef([SAGE_ECM_MINVER])
+], [
+    # REQUIRED-check is empty
+], [
+    # PRE - always perform
+    ECMBIN=ecm
+], [
+    # POST - always perform
     AC_SUBST(SAGE_ECMBIN, $ECMBIN)
 ])