test: also edit /etc/os-release if it's not a symlink when patching /usr/lib/os-release (#38628)

mkosi patches up /etc/os-release to add local IDs and fixup certain
issues, so when tests patch /usr/lib/ on the fly, copy to the version in
/etc/ too to avoid test failures when querying

6370s 10/98 systemd:integration-tests / TEST-07-PID1 FAIL 31.03s exit
status 1
6370s 25/98 systemd:integration-tests / TEST-29-PORTABLE FAIL 12.76s
exit status 1
6370s 33/98 systemd:integration-tests / TEST-43-PRIVATEUSER-UNPRIV FAIL
6.57s exit status 1
6370s 37/98 systemd:integration-tests / TEST-50-DISSECT FAIL 16.97s exit
status 1

This is particularly an issue when running these tests on debian unstable,
where mkosi has to fixup os-release to make it valid and avoid further
breakages:

https://github.com/systemd/mkosi/blob/main/mkosi/distributions/debian.py#L234

mkosi/mkosi.images/minimal-0/mkosi.postinst

@@ -8,4 +8,7 @@ cat >>"$BUILDROOT/usr/lib/os-release" <<EOF
 MARKER=1
 PORTABLE_PREFIXES=app0 minimal minimal-app0
 EOF
+if [ ! -L "$BUILDROOT/etc/os-release" ]; then
+    cp "$BUILDROOT/usr/lib/os-release" "$BUILDROOT/etc/os-release"
+fi
 cp "$BUILDROOT/usr/lib/systemd/system/minimal-app0.service" "$BUILDROOT/usr/lib/systemd/system/minimal-app0-foo.service"

mkosi/mkosi.images/minimal-1/mkosi.postinst

@@ -8,4 +8,7 @@ cat >>"$BUILDROOT/usr/lib/os-release" <<EOF
 MARKER=2
 PORTABLE_PREFIXES=app0 minimal minimal-app0
 EOF
+if [ ! -L "$BUILDROOT/etc/os-release" ]; then
+    cp "$BUILDROOT/usr/lib/os-release" "$BUILDROOT/etc/os-release"
+fi
 cp "$BUILDROOT/usr/lib/systemd/system/minimal-app0.service" "$BUILDROOT/usr/lib/systemd/system/minimal-app0-bar.service"