Fix TypeScript errors in the build

[leerob]

The build is currently failing with:

▲  rust/ yarn build
yarn run v1.22.11
$ tsc
src/index.ts:253:9 - error TS2322: Type '{ PATH: string; RUSTFLAGS: string; TZ?: string | undefined; }' is not assignable to type 'Record<string, string>'.
  Property 'TZ' is incompatible with index signature.
    Type 'string | undefined' is not assignable to type 'string'.
      Type 'undefined' is not assignable to type 'string'.

253   const rustEnv: Record<string, string> = {
            ~~~~~~~

src/index.ts:280:11 - error TS2322: Type '{ PATH: string; RUSTFLAGS: string; TZ?: string | undefined; }' is not assignable to type 'Record<string, string>'.
  Property 'TZ' is incompatible with index signature.
    Type 'string | undefined' is not assignable to type 'string'.
      Type 'undefined' is not assignable to type 'string'.

280     const rustEnv: Record<string, string> = {
              ~~~~~~~


Found 4 errors.

[icyJoseph]

I fixed this by making the process.env type fit into the RustEnv type, but I reckon the project uses TS 3.x.x, and would like to upgrade it to 4.4, not sure if there are any limitations?

[leerob]

Thank you @icyJoseph ❤️ Created #52