fix(test): pull elf fixture build tools from archive.debian.org (#5264)

bullseye went EOL and deb.debian.org dropped it, so the elf executable
fixture image stopped building and took the whole unit test job with it
(fixture generation runs before any test does). main has been red on this
since 2026-09-08.

two separate breakages, both handled here:

- the base repo moved to `archive.debian.org`, whose Release files are
  permanently expired, so `Acquire::Check-Valid-Until=false` is required

- `bullseye-security` is worse off: its index is still served but every
  package in the pool behind it 404s, and it has not landed on the archive
  yet. dropping it is the only option right now. these are throwaway
  fixture build tools, so base bullseye versions are fine.

verified by rebuilding the image and running the fixture `verify` target,
which still reports `PASS: all security features accounted for`.

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

syft/file/cataloger/executable/testdata/elf/Dockerfile

@@ -1,6 +1,12 @@
 FROM gcc:9.5.0
 
-RUN apt update -y && apt install -y clang cmake git make m4 pkg-config zlib1g-dev
+# bullseye is EOL: deb.debian.org dropped it, so pull from archive.debian.org (whose Release files are
+# permanently expired, hence Check-Valid-Until). bullseye-security is deliberately omitted -- its index is
+# still served but the pool behind it 404s, and it has not been moved to the archive yet. These are throwaway
+# fixture build tools, so base bullseye package versions are fine.
+RUN printf 'deb http://archive.debian.org/debian bullseye main\ndeb http://archive.debian.org/debian bullseye-updates main\n' > /etc/apt/sources.list \
+    && apt -o Acquire::Check-Valid-Until=false update -y \
+    && apt install -y clang cmake git make m4 pkg-config zlib1g-dev
 
 ## from https://github.com/runsafesecurity/selfrando/blob/tb-v0.4.2/docs/linux-build-instructions.md
 #RUN git clone https://github.com/runsafesecurity/selfrando.git && \