gh-37764: ECM-related tests fail after an incremental build

### TL;DR:

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: ''

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

URL: #37764
Reported by: Volker Braun
Reviewer(s): Matthias Köppe

build/pkgs/configure/checksums.ini

@@ -1,4 +1,4 @@
 tarball=configure-VERSION.tar.gz
-sha1=8cfc13031178334306e8a2745ae9d822790f8029
-md5=25296e38dfa8eaf36644f8e6ff038c9e
-cksum=991738162
+sha1=0223519e28f35b4163a994a4fd6bbed3fdbffdbe
+md5=53f144a31102971db15954c7229d252e
+cksum=4202935372

build/pkgs/configure/package-version.txt

@@ -1 +1 @@
-850ed07c17579aa4c092159ca7042545ba6b3213
+8b9a3b1f938364d711320a4e017cdf50dd342eb7

build/pkgs/ecm/spkg-configure.m4

@@ -1,6 +1,5 @@
-SAGE_SPKG_CONFIGURE([ecm], [
+SAGE_SPKG_CONFIGURE([ecm], [dnl 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 +34,9 @@ SAGE_SPKG_CONFIGURE([ecm], [
         ], [sage_spkg_install_ecm=yes])
     ])
     m4_popdef([SAGE_ECM_MINVER])
-    AC_SUBST(SAGE_ECMBIN, $ECMBIN)
+], [dnl REQUIRED-check is empty
+], [dnl PRE - always perform
+    ECMBIN=ecm
+], [dnl POST - always perform
+    AC_SUBST([SAGE_ECMBIN], ["$ECMBIN"])
 ])