Ensure Send/Sync is not implemented for std::env::Vars{,Os}

[Mark-Simulacrum]

On some platforms, it looks like these are not currently Send/Sync. We have a negative impl in env/common.rs. This is a breaking change on a bunch of targets, but it makes it less likely that someone unintentionally/accidentally depends on this. This is a potentially partial list but should be fairly complete. These are all platforms where these previously were unsupported, so this is relatively unlikely to actually break them.

aarch64-unknown-none, aarch64-unknown-none-softfloat, armv7a-none-eabi, armv7a-none-eabihf, armv7r-none-eabi, armv7r-none-eabihf, armv8r-none-eabihf, loongarch64-unknown-none,
loongarch64-unknown-none-softfloat, nvptx64-nvidia-cuda, riscv32i-unknown-none-elf, riscv32im-unknown-none-elf, riscv32imac-unknown-none-elf, riscv32imafc-unknown-none-elf, riscv32imc-unknown-none-elf, riscv64gc-unknown-none-elf, riscv64imac-unknown-none-elf, s390x-unknown-none-softfloat, thumbv6m-none-eabi, thumbv7a-none-eabi, thumbv7a-none-eabihf, thumbv7em-none-eabi, thumbv7em-none-eabihf, thumbv7m-none-eabi, thumbv7r-none-eabi, thumbv7r-none-eabihf, thumbv8m.base-none-eabi, thumbv8m.main-none-eabi, thumbv8m.main-none-eabihf, thumbv8r-none-eabihf, wasm32-unknown-unknown, wasm32v1-none, x86_64-unknown-none

cc #154517

[rustbot]

@Mark-Simulacrum: no appropriate reviewer found, use r? to override

[Amanieu]

@rfcbot merge libs-api

[rust-rfcbot]

Team member @Amanieu has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @the8472

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

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[Mark-Simulacrum]

Just to clarify @nia-e since I see in the meeting notes, in case it affects anyone's FCP box:

Nia: This one is trickier because it's removing a negative impl.

This is not removing a negative impl, it's adding that negative impl to all targets by moving it out of sys/ and into the public type's API.