testing/rust: build with Cargo specified in stage0.txt

This is not necessary, but better...

testing/rust/APKBUILD

@@ -3,8 +3,8 @@
 # Maintainer: Jakub Jirutka <jakub@jirutka.cz>
 pkgname=rust
 pkgver=1.16.0
-# Git revision of prebuilt cargo to use for bootstrapping.
-_cargo_gitrev=f9e54817e53c7b9845cc7c1ede4c11e4d3e42e36  # 0.17.0
+# Git revision of prebuilt Cargo to use for bootstrapping, from src/stage0.txt.
+_cargo_gitrev=6e0c18cccc8b0c06fba8a8d76486f81a792fb420
 pkgrel=0
 pkgdesc="The Rust Programming Language (compiler)"
 url="http://www.rust-lang.org"
@@ -53,6 +53,11 @@ prepare() {
 
 	cd "$builddir"
 
+	local stage0_cargo=$(sed -En 's/^cargo:\s*(\S+)$/\1/p' src/stage0.txt)
+	if [ "$_cargo_gitrev" != "$stage0_cargo" ]; then
+		error "Update Cargo revision to: $stage0_cargo"; return 1
+	fi
+
 	# Don't set wrong LD_LIBRARY_PATH, we will rather set it manually when
 	# invoking make.
 	sed -i /LD_LIBRARY_PATH/d src/bootstrap/bootstrap.py
@@ -170,7 +175,7 @@ _mv() {
 sha512sums="096b1b7406be9bc61161bb7cdd2061f2bc2174c161a31f4ed6ceecf7fc379f315fc2f7cb9f6c134ea4f8519c27bf6e5a3f712cf1e56d5785831d8c8374eb0ba5  rustc-1.16.0-src.tar.gz
 e9b10d58ae5b51b09dd31a1dbf7367917bd40a05ecb4ba3e0e7ac229a0352d3ceb77de80f7c0120f553bd7904644da6bf973dbc32850b12dc91f3974846d4164  rustc-1.16.0-x86_64-unknown-linux-musl.tar.gz
 54467213b2824112dec6d5a132ab01d69617a5ccaa9db15e5c8fad55dea5eb31b1b7c9ca07878901d75edd45dc84c726913bf651f3435c480a79703c074b5d90  rust-std-1.16.0-x86_64-unknown-linux-musl.tar.gz
-97698f6b9a1eae6e181dc480599095f0aa5fcf85cdc3dc6b5340a20e1e42f585188c06074c1e35cff4393a78c8502c81d8852fcf7da674e0a7313f88155feb56  cargo-f9e54817e53c7b9845cc7c1ede4c11e4d3e42e36-x86_64-unknown-linux-musl.tar.gz
+4df33353b6be1af822220ca03f9ec5a3b6421c9b10ec17b864be586ba495c6210bac70d904b0e0b35eff62ef2c9a681ded526e5cbd37e326dc3146742aa659de  cargo-6e0c18cccc8b0c06fba8a8d76486f81a792fb420-x86_64-unknown-linux-musl.tar.gz
 2002faf08cca3be702f25618bc17453c5e76644a6ff6b0739b642c2c0f8536ec7f09c98182aff6b2dcc87c77b0929f2f8f732b8242a8f80d94b3824c8cd0d0fe  dont-require-filecheck.patch
 a87ffe129debcb9fa958c6d007ffa4a87927890030e0694ee5ab1929ea0c5a48dad86b4e8b71550dcfeda29b644dbb23d8b232bda62439f198a404cf658c68b0  support-dynamically-linked-musl.patch
 57ea7651bc0b0fa5340b9c5de0492ddc8bfaf4f1003298bf27dea57ee562cdd0f3947e091b154388535ce511e001e052f12d48805f7ffece7f04178ad409623f  allow-crt-static-on-stable.patch