build: force build stage image to buster

paritytech/ci-linux:production was recently switched from buster to bullseye,

added RN-silent label

requested review from @moul

approved this merge request

The build fails with:

  Downloaded ring v0.16.20
   Compiling cranelift-isle v0.88.2
   Compiling duniter v0.3.0 (/builds/nodes/rust/duniter-v2s)
   Compiling gdev-runtime v3.0.0 (/builds/nodes/rust/duniter-v2s/runtime/gdev)
error: failed to run custom build command for `gdev-runtime v3.0.0 (/builds/nodes/rust/duniter-v2s/runtime/gdev)`
Caused by:
  process didn't exit successfully: `/builds/nodes/rust/duniter-v2s/target/debug/build/gdev-runtime-c089ee2e291ae98a/build-script-build` (exit status: 1)
  --- stderr
  /builds/nodes/rust/duniter-v2s/target/debug/build/gdev-runtime-c089ee2e291ae98a/build-script-build: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.29' not found (required by /builds/nodes/rust/duniter-v2s/target/debug/build/gdev-runtime-c089ee2e291ae98a/build-script-build)

Could this be cache related?

EDIT: Yes it is cache related. The release build went fine. @moul Any way to clear the cache on the redshift-host runner?

resolved all threads

merged