script/release.sh: add -a to force rebuild

https://golang.org/cmd/go/#hdr-Build_and_test_caching says:

> If you have made changes to the C libraries on your system, you will
> need to clean the cache explicitly or else use the -a build flag
> (see 'go help build') to force rebuilding of packages that depend
> on the updated C libraries.

This means that:

1. We need to either 'go clean -cache' or 'go build -a' when building
   the release binary. Adding '-a' seems less intrusive / more focused.

2. The check for existing libseccomp.a (added by commit d748280aa)
   is no longer needed.

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

script/release.sh

@@ -30,25 +30,6 @@ function build_project() {
 	# Due to libseccomp being LGPL we must include its sources,
 	# so download, install and build against it.
 
-	# Sanity check: check that the build won't use distro-provided
-	# libseccomp.a. If this is the case, explain and abort.
-	#
-	# Note go is not used here as it caches the build heavily,
-	# including, apparently, the results of pkg-config.
-	# shellcheck disable=SC2046
-	if read -ra flags < <(pkg-config --libs --cflags libseccomp 2>/dev/null) &&
-		cat <<EOF | gcc -static -x c -o /dev/null - "${flags[@]}"; then
-#include <seccomp.h>
-int main(void) { seccomp_version(); return 0; }
-EOF
-		set +x
-		echo >&2
-		echo "Distro-provided libseccomp static library is installed." >&2
-		echo "Unable to build a static binary against own libsecomp." >&2
-		echo "Please uninstall libseccomp-static to fix." >&2
-		exit 1
-	fi
-
 	local libseccomp_ver='2.5.1'
 	local tarball="libseccomp-${libseccomp_ver}.tar.gz"
 	local prefix
@@ -62,7 +43,10 @@ EOF
 	)
 	mv "$tarball"{,.asc} "$builddir"
 
-	make -C "$root" PKG_CONFIG_PATH="${prefix}/lib/pkgconfig" COMMIT_NO= static
+	# Add -a to go build flags to make sure it links against
+	# the provided libseccomp, not the system one (otherwise
+	# it can reuse cached pkg-config results).
+	make -C "$root" PKG_CONFIG_PATH="${prefix}/lib/pkgconfig" COMMIT_NO= EXTRA_FLAGS="-a" static
 	rm -rf "$prefix"
 	mv "$root/$project" "$1"
 }