Revert default behavior of cargo build

My PR #3213 carelessly changed the default behavior of cargo build.
This commit restored the old behavior while still fixing
cross-compilation. For this, instead of relying on the default
target matching the compilation host, we instead explicitly read
the host target from the HOST variable that cargo sets
for build scripts.

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

.cargo/config

@@ -1,5 +1,6 @@
 [build]
 target-dir = "build/cargo_target"
+target = "x86_64-unknown-linux-musl"
 
 [net]
 git-fetch-with-cli = true

src/vmm/build.rs

@@ -51,6 +51,11 @@ fn main() {
 
 // Run seccompiler with the given arguments.
 fn run_seccompiler_bin(json_path: &str, out_path: &str) {
+    // We have a global `target` directive in our .cargo/config file specifying x86_64 architecture.
+    // However, seccompiler-bin has to be compiled for the host architecture. Without this, cargo
+    // would produce a x86_64 binary on aarch64 host, causing this compilation step to fail as such
+    // a binary would not be executable.
+    let host_arch = env::var("HOST").expect("Could not determine compilation host");
     let target_arch = env::var("CARGO_CFG_TARGET_ARCH").expect("Missing target arch.");
 
     // Command for running seccompiler-bin
@@ -60,6 +65,8 @@ fn run_seccompiler_bin(json_path: &str, out_path: &str) {
         "-p",
         "seccompiler",
         "--verbose",
+        "--target",
+        &host_arch,
         // We need to specify a separate build directory for seccompiler-bin. Otherwise, cargo will
         // deadlock waiting to acquire a lock on the build folder that the parent cargo process is
         // holding.