Filter out cargo dependencies for unsupported platforms

[vashirov]

Issue Description
cargo vendor downloads all dependencies including for Windows. For example, our vendor directory includes winapi crate and libraries required by ansi_term:

$ du -hs vendor/winapi* 
7.6M    vendor/winapi
52M     vendor/winapi-i686-pc-windows-gnu
54M     vendor/winapi-x86_64-pc-windows-gnu

cargo vendor doesn't currently support filtering out platform-specific dependencies: rust-lang/cargo#7058
But there is a cargo-vendor-filterer project:
rust-lang/cargo#7058 (comment)
https://github.com/coreos/cargo-vendor-filterer

It should shave about 5MB from the size of the compressed released tarball:

$ du 389-ds-base-latest.tar.bz2 
14M     389-ds-base-latest.tar.bz2

vs.

$ du 389-ds-base-latest_filtered.tar.bz2
8.6M    389-ds-base-latest_filtered.tar.bz2

[vashirov]

One issue I found is with Cargo.lock file:

SLAPD_HEADER_DIR=/home/vashirov/src/ds/rpmbuild/BUILD/389-ds-base-2.3.0.202210051402gitcf4c82c21/ \
        cargo rustc --locked --offline --manifest-path=./src/librnsslapd/Cargo.toml \
        --release --verbose -- -C debuginfo=2 
    Blocking waiting for file lock on package cache
error: the lock file /home/vashirov/src/ds/rpmbuild/BUILD/389-ds-base-2.3.0.202210051402gitcf4c82c21/src/Cargo.lock needs to be updated but --locked was passed to prevent this
If you want to try to generate the lock file without accessing the network, remove the --locked flag and use --offline instead.
make[1]: *** [Makefile:12983: /home/vashirov/src/ds/rpmbuild/BUILD/389-ds-base-2.3.0.202210051402gitcf4c82c21/rs/rnsslapd/release/librnsslapd.a] Error 101
make[1]: *** Waiting for unfinished jobs....
error: the lock file /home/vashirov/src/ds/rpmbuild/BUILD/389-ds-base-2.3.0.202210051402gitcf4c82c21/src/Cargo.lock needs to be updated but --locked was passed to prevent this
If you want to try to generate the lock file without accessing the network, remove the --locked flag and use --offline instead.
make[1]: *** [Makefile:12972: /home/vashirov/src/ds/rpmbuild/BUILD/389-ds-base-2.3.0.202210051402gitcf4c82c21/rs/rslapd/release/librslapd.a] Error 101
make[1]: Leaving directory '/home/vashirov/src/ds/rpmbuild/BUILD/389-ds-base-2.3.0.202210051402gitcf4c82c21'
error: Bad exit status from /var/tmp/rpm-tmp.MYjwuT (%build)

If I remove --locked from the build options, it builds fine. Or I can modify the Cargo.lock file to remove the dependencies that were filtered out. But next time someone runs cargo update or cargo vendor without cargo vendor-filterer, it will overwrite Cargo.lock file to again include those dependencies.

Another issue is missing support for --manifest-path: coreos/cargo-vendor-filterer#31
It's not critical and can be worked around.

So until cargo vendor supports filtering natively, we might run into issues like these.

Here's the diff if someone wants to play with this:

diff --git a/rpm.mk b/rpm.mk                                                                                                                       
index ed72ee567..c3b40c7ed 100644
--- a/rpm.mk
+++ b/rpm.mk
@@ -38,9 +38,9 @@ update-cargo-dependencies:
  
 download-cargo-dependencies:
        cargo update --manifest-path=./src/Cargo.toml
-       cargo vendor --manifest-path=./src/Cargo.toml
-       cargo fetch --manifest-path=./src/Cargo.toml
-       tar -czf vendor.tar.gz vendor
+       cd src; cargo vendor-filterer --platform=x86_64-unknown-linux-gnu --prefix=vendor --format=tar.gz
+       mv src/vendor.tar.gz .
+       tar -xf vendor.tar.gz
  
 bundle-rust:
        python3 rpm/bundle-rust-downstream.py ./src/Cargo.lock $(DS_SPECFILE) ./vendor