libafl_bolts: fix musl build (#1421)

despite being present in the headers, getcontext is not implemented in
musl libc, most likely due to the fact it s a deprecated interface.
The only way around is having the third party libucontext apk package
installed.

libafl_bolts/build.rs

@@ -2,9 +2,13 @@
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:rustc-cfg=nightly");
+    #[cfg(target_env = "musl")]
+    println!("cargo:rustc-link-lib=ucontext");
 }
 
 #[rustversion::not(nightly)]
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
+    #[cfg(target_env = "musl")]
+    println!("cargo:rustc-link-lib=ucontext");
 }