failed to select a version for the requirement platforms = "^0.2.1"

[HamzaYasin1]

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• This is not a support question.

Description of bug

Hi Folks,
I'm using substrate 2.0.0 as of baseline code, it was working fine a week ago but than it started giving this issue:

          error: failed to run custom build command for `node-template-runtime v2.0.0               
          Caused by:
            process didn't exit successfully: `/Users/dev/Applications/Documents/blockchain/substrate-frame-pallets- 
           modules/target/debug/build/node-template-runtime-aa7158538ef102e9/build-script-build` (exit code: 1)
            --- stderr
                Updating crates.io index
            error: failed to download `serde v1.0.143`
          
            Caused by:
              unable to get packages from source
          
            Caused by:
              failed to parse manifest at `/Users/dev/Applications/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/serde-
             1.0.143/Cargo.toml`
          
            Caused by:
              feature `resolver` is required
          
              consider adding `cargo-features = ["resolver"]` to the manifest

I searched this issue and i found that its open on serde.rs2255
i did many things, like updating or downgrading toolchain, but when i removed the lock file and deleted the targets so this started giving below error. i couldn't see any platforms dependency in cargo tree in my project.

      error: failed to select a version for the requirement `platforms = "^0.2.1"`
            candidate versions found which didn't match: 3.0.1, 3.0.0, 2.0.0, ...
            location searched: crates.io index
            required by package `substrate-build-script-utils v2.0.0`
                ... which is depended on by `my-chain v2.0.0 

Steps to reproduce

• Using substrate 2.0.0
• cargo +nightly-2020-10-05 build