Fix a %buildroot regression on an early %__spec_install_pre %global override

The Fedora kernel.spec overrides %__spec_install_pre with this very
early in the spec:

	%global __spec_install_pre %{___build_pre}

This happens long before Name and all that is defined, so half of
the environment setup in %___build_pre just falls through with
unexpanded macros as the values (eg RPM_PACKAGE_NAME=%{NAME}) at this
stage, and get their real values on the re-expansion before the actual
scriptlet execution. But, due to pre-historical reasons, RPM_BUILD_ROOT
setting is conditional, depending on whether %buildroot is defined or
not. It used to be defined in the main macros file, so RPM_BUILD_ROOT
would also get that template-style value at this point in the spec,
but since 9d35c8df49 it no longer is.
And that causes RPM_BUILD_ROOT to be *dropped* from the pre-environment
"template" at this early stage of spec parsing.

Add a simple reproducer test, this level of meddling in packages is of
playing with fire and we may not be able to guarantee it always stays
this way, but at least we'll be aware of possible changes in behavior.

Commit 0a4e182e40 from 1998 is the first
macro version of this conditional that came back to bite us. It
should've been removed in 9d35c8df49.

Reported in https://bugzilla.redhat.com/show_bug.cgi?id=2284036

macros.in

@@ -770,8 +770,8 @@ Supplements:   (%{name} = %{version}-%{release} and langpacks-%{1})\
   unset CDPATH DISPLAY ||:\
   unset DEBUGINFOD_URLS ||:\
   unset RPM_CONFIG_DIR ||:\
-  %{?buildroot:RPM_BUILD_ROOT=\"%{buildroot}\"\
-  export RPM_BUILD_ROOT}\
+  RPM_BUILD_ROOT=\"%{buildroot}\"\
+  export RPM_BUILD_ROOT\
   %{?_javaclasspath:CLASSPATH=\"%{_javaclasspath}\"\
   export CLASSPATH}\
   PKG_CONFIG_PATH=\"${PKG_CONFIG_PATH}:%{_libdir}/pkgconfig:%{_datadir}/pkgconfig\"\

tests/data/SPECS/install-hack.spec

@@ -0,0 +1,20 @@
+# Mimic what fedora kernel.spec does. Whether this is a legitimate
+# thing for a package to or not is another question, but at least we'll
+# be aware if the behavior changes.
+%global __spec_install_pre %{___build_pre}
+
+Summary: install scriptlet section override hack
+Name: install-hack
+Version: 1.0
+Release: 1
+License: GPL
+Buildarch: noarch
+
+%description
+%{summary}
+
+%install
+mkdir -p $RPM_BUILD_ROOT/usr/local/bin
+
+%files
+/usr/local/bin/

tests/rpmbuild.at

@@ -3277,3 +3277,15 @@ BUILD_POST noarch noarch noarch
 ],
 [])
 RPMTEST_CLEANUP
+
+AT_SETUP([rpmbuild install_pre override])
+AT_KEYWORDS([build])
+RPMDB_INIT
+RPMTEST_CHECK([
+runroot rpmbuild -bb /data/SPECS/install-hack.spec
+],
+[0],
+[ignore],
+[ignore])
+RPMTEST_CLEANUP
+