message: Clean up appveyor config.
  Use correct version for minimal-versions check.
  Don't bother downloading OTHER_TARGET for minimal-versions.
  Remove an old/unused env var.
diff: @@ -2,10 +2,7 @@ environment:
  matrix:
  - TARGET: x86_64-pc-windows-msvc
  OTHER_TARGET: i686-pc-windows-msvc
  - MAKE_TARGETS: test-unit-x86_64-pc-windows-msvc
  - TARGET: x86_64-pc-windows-msvc
  - OTHER_TARGET: i686-pc-windows-msvc
  - MAKE_TARGETS: test-unit-x86_64-pc-windows-msvc
  MINIMAL_VERSIONS: true
  CFG_DISABLE_CROSS_TESTS: 1
  @@ -15,7 +12,7 @@ install:
  - rustup-init.exe -y --default-host x86_64-pc-windows-msvc --default-toolchain nightly
  - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin
  - if defined MINIMAL_VERSIONS rustup toolchain install 1.28.0
  - - rustup target add %OTHER_TARGET%
  + - if defined OTHER_TARGET rustup target add %OTHER_TARGET%
  - rustc -V
  - cargo -V
  - git submodule update --init
  @@ -28,5 +25,5 @@ test_script:
  # we don't have ci time to run the full `cargo test` with `minimal-versions` like
  # - if defined MINIMAL_VERSIONS cargo +nightly generate-lockfile -Z minimal-versions && cargo +stable test
  # so we just run `cargo check --tests` like
  - - if defined MINIMAL_VERSIONS cargo +nightly generate-lockfile -Z minimal-versions && cargo check --tests
  + - if defined MINIMAL_VERSIONS cargo +nightly generate-lockfile -Z minimal-versions && cargo +1.28.0 check --tests
  - if NOT defined MINIMAL_VERSIONS cargo test
  