Remove some rerun-if-env-changed lines from build.rs. (#576)

Now that we appear to have a known-working fix for #526, we can
investigate relaxing the fix. If #526 reappears, we now have a
known-working state we can quickly revert to.

Reports in #562 and #575 are that having rerun-if-env-changed=RUSTC
and rerun-if-env-changed=RUSTC_WRAPPER are triggering spurious rebuilds
in some situations, so remove these. In theory, cargo should already be
aware of these environment variables. In practice, there were reports of
build failures which had the appearance of using an older version of
rustc with build.rs output from a newer version of rustc, and it wasn't
clear what was happening. But, it's possible that #563 fixes the issue
properly. So let's try removing these rerun-if-env-changed lines, and
see if the problem resurfaces.

And, rerun-if-env-changed=TARGET is explicitly documented as being
unnecessary [here].

[here]: https://doc.rust-lang.org/cargo/reference/build-scripts.html

Author: sunfishcode

build.rs

@@ -114,11 +114,6 @@ fn main() {
     println!("cargo:rerun-if-env-changed=CARGO_FEATURE_USE_LIBC");
     println!("cargo:rerun-if-env-changed=CARGO_FEATURE_RUSTC_DEP_OF_STD");
     println!("cargo:rerun-if-env-changed=CARGO_CFG_MIRI");
-    println!("cargo:rerun-if-env-changed=CARGO_ENCODED_RUSTFLAGS");
-    println!("cargo:rerun-if-env-changed=RUSTC");
-    println!("cargo:rerun-if-env-changed=TARGET");
-    println!("cargo:rerun-if-env-changed=RUSTC_WRAPPER");
-    println!("cargo:rerun-if-env-changed=PROFILE");
 }
 
 /// Link in the desired version of librustix_outline_{arch}.a, containing the