Since commit 1c5976ef0f7a the kernel deregisters all binfmt_misc handler
when the binfmt_misc filesystem is unmounted.  The build script has
this:

# RPM specific post checks are in /usr/lib/build/checks
if test -n "$RPMS" -a -d "$BUILD_ROOT/usr/lib/build/checks" ; then
    # workaround for broken 13.1 check scripts which umount /proc
    if test -n "$RUNNING_IN_VM" -a "$BUILD_ROOT" = / ; then
        umount -n $BUILD_ROOT/proc/sys/fs/binfmt_misc 2>/dev/null
    fi

This means that after this point the emulation no longer works.

[  272s] ... checking for files with abuild user/group
[  272s] /.build/build: line 1922: /bin/env: cannot execute binary file: Exec format error
[  272s] /.build/build: line 1926: /bin/rpm: cannot execute binary file: Exec format error
[  272s] /.build/build: line 1926: /bin/rpm: cannot execute binary file: Exec format error
[  272s] /.build/build: line 1926: /bin/rpm: cannot execute binary file: Exec format error
[  272s] /.build/build: line 1926: /bin/rpm: cannot execute binary file: Exec format error
[  272s] /.build/build: line 1926: /bin/rpm: cannot execute binary file: Exec format error
[  272s] /.build/build: line 1926: /bin/rpm: cannot execute binary file: Exec format error
[  272s] /.build/build: line 1926: /bin/rpm: cannot execute binary file: Exec format error
[  272s] /.build/build: line 1926: /bin/rpm: cannot execute binary file: Exec format error
[  272s] /.build/build: line 1926: /bin/rpm: cannot execute binary file: Exec format error
[  272s] /.build/build: line 1926: /bin/rpm: cannot execute binary file: Exec format error

According to the commit, this should be reported as a regression so that this behaviour is reverted for the initial binfmt_misc mount.