remove build.rs, use CARGO_MANIFEST_DIR env directly

[lblasc]

CARGO_MANIFEST_DIR path can vary when crate is compiled and later used in crates like rust-headless-chrome.

This resolves issue I'm having with nix, most likely bazel will have the same issue:

thread 'main' panicked at src/compile.rs:1273:50:
called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }
stack backtrace:
   0: rust_begin_unwind
   1: aws_lc_0_28_0_aes_gcm_decrypt_avx512
             at /build/aws-lc-sys-0.28.0/aws-lc/generated-src/linux-x86_64/crypto/fipsmodule/aesni-gcm-avx512.S:20266:0
   2: aws_lc_0_28_0_aes_gcm_decrypt_avx512
             at /build/aws-lc-sys-0.28.0/aws-lc/generated-src/linux-x86_64/crypto/fipsmodule/aesni-gcm-avx512.S:20698:0
   3: aws_lc_0_28_0_aes_gcm_decrypt_avx512
             at /build/aws-lc-sys-0.28.0/aws-lc/generated-src/linux-x86_64/crypto/fipsmodule/aesni-gcm-avx512.S:58398:0
   4: aws_lc_0_28_0_aes_gcm_decrypt_avx512
             at /build/aws-lc-sys-0.28.0/aws-lc/generated-src/linux-x86_64/crypto/fipsmodule/aesni-gcm-avx512.S:58510:0
   5: aws_lc_0_28_0_aes_gcm_decrypt_avx512
             at /build/aws-lc-sys-0.28.0/aws-lc/generated-src/linux-x86_64/crypto/fipsmodule/aesni-gcm-avx512.S:22940:0
   6: build_script_build::main
             at ./build.rs:4:5
   7: core::ops::function::FnOnce::call_once
             at /build/rustc-1.86.0-src/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.