make release: build/include libseccomp

libseccomp is LGPL, meaning if we statically link it, we have to include
the source code of the library.

Amend "make release" to download and build libseccomp, build runc
against it, and include its sources into the release directory.

The only caveat is I found no way to stop go build from using the
stock (distro-provided) libseccomp.a, so the script checks that
the stock libseccomp.a is not available, and aborts otherwise.

While at it:
 - enable shellcheck for script/release.sh
 - remove libseccomp installation from the gha job
 - add dependecies needed for libseccomp build to the gha job

[v2: also include libseccomp .asc file]
[v3: rebase]

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.github/workflows/validate.yml

@@ -153,7 +153,7 @@ jobs:
     - name: install deps
       run: |
         sudo apt -qq update
-        sudo apt -qq install libseccomp-dev
+        sudo apt -qq install gperf
     - name: make release
       run: make release
     - name: upload artifacts