selinux: use safe procfs API for labels

Due to the sensitive nature of these fixes, it was not possible to
submit these upstream and vendor the upstream library. Instead, this
patch uses a fork of github.com/opencontainers/selinux, branched at
commit opencontainers/selinux@879a755db5.

In order to permit downstreams to build with this patched version, a
snapshot of the forked version has been included in
internal/third_party/selinux. Note that since we use "go mod vendor",
the patched code is usable even without being "go get"-able. Once the
embargo for this issue is lifted we can submit the patches upstream and
switch back to a proper upstream go.mod entry.

Also, this requires us to temporarily disable the CI job we have that
disallows "replace" directives.

Fixes: GHSA-cgrx-mc8f-2prm CVE-2025-52881
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

go.sum

@@ -48,8 +48,6 @@ github.com/opencontainers/cgroups v0.0.5 h1:DRITAqcOnY0uSBzIpt1RYWLjh5DPDiqUs4fY
 github.com/opencontainers/cgroups v0.0.5/go.mod h1:oWVzJsKK0gG9SCRBfTpnn16WcGEqDI8PAcpMGbqWxcs=
 github.com/opencontainers/runtime-spec v1.2.2-0.20250818071321-383cadbf08c0 h1:RLn0YfUWkiqPGtgUANvJrcjIkCHGRl3jcz/c557M28M=
 github.com/opencontainers/runtime-spec v1.2.2-0.20250818071321-383cadbf08c0/go.mod h1:jwyrGlmzljRJv/Fgzds9SsS/C5hL+LL3ko9hs6T5lQ0=
-github.com/opencontainers/selinux v1.12.0 h1:6n5JV4Cf+4y0KNXW48TLj5DwfXpvWlxXplUkdTrmPb8=
-github.com/opencontainers/selinux v1.12.0/go.mod h1:BTPX+bjVbWGXw7ZZWUbdENt8w0htPSrlgOOysQaU62U=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/rogpeppe/go-internal v1.11.0 h1:cWPaGQEPrBb5/AsnsZesgZZ9yb1OQ+GOISoDNXVBh4M=