Use CARGO_PROFILE_RELEASE_LTO to fix bindings build on new cargo

Newer versions of cargo broke `cargo rustc -- -Clto` by passing
`-Cembed-bitcode=no` even in `--release`. Its somewhat unclear why
this is still the case on latest cargo given it broke, at least,
compilation of Firefox, as discovered by Val at [1]. We take the
approach they used there, even though they later walked it back [2]
but the issues noted there, especially in [3] don't seem
particularly concerning in our case.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1640982
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=1654465
[3] https://bugzilla.mozilla.org/show_bug.cgi?id=1654465#c5

Author: TheBlueMatt

genbindings.sh

@@ -167,7 +167,7 @@ else
 fi
 
 # Now build with LTO on on both C++ and rust, but without cross-language LTO:
-cargo rustc -v --release -- -C lto
+CARGO_PROFILE_RELEASE_LTO=true cargo rustc -v --release -- -C lto
 clang++ -std=c++11 -Wall -flto -O2 -pthread demo.cpp target/release/libldk.a -ldl
 echo "C++ Bin size and runtime with only RL (LTO) optimized:"
 ls -lha a.out
@@ -179,7 +179,7 @@ if [ "$HOST_PLATFORM" != "host: x86_64-apple-darwin" -a "$CLANGPP" != "" ]; then
 	# or Ubuntu packages). This should work fine on Distros which do more involved
 	# packaging than simply shipping the rustup binaries (eg Debian should Just Work
 	# here).
-	cargo rustc -v --release -- -C linker-plugin-lto -C lto -C link-arg=-fuse-ld=lld
+	CARGO_PROFILE_RELEASE_LTO=true cargo rustc -v --release -- -C linker-plugin-lto -C lto -C link-arg=-fuse-ld=lld
 	$CLANGPP -Wall -std=c++11 -flto -fuse-ld=lld -O2 -pthread demo.cpp target/release/libldk.a -ldl
 	echo "C++ Bin size and runtime with cross-language LTO:"
 	ls -lha a.out