Allow overriding popcorn id with POPCORN_SUBMITTER_ID env arg

[Jack-Khuu]

When submitting via proxy or in a setting with multiple accounts it is difficult to switch submitter identites (currently always reads from .popcorn.yaml)

This PR adds POPCORN_SUBMITTER_ID as an env flag override

Build

./build.sh

Normal run

> ./target/release/popcorn-cli submissions list --leaderboard matmul_v2

ID       Leaderboard          File                 Time                 GPU(s)       Status          Score
---------------------------------------------------------------------------------------------------------

Override

> POPCORN_SUBMITTER_ID=0987654 ./target/release/popcorn-cli submissions list --leaderboard matmul_v2

**Application error: Server returned status 401 Unauthorized: Invalid or unauthorized auth header elaine**

>   POPCORN_SUBMITTER_ID=$(grep 'cli_id:' ~/.popcorn.yaml | awk '{print $2}') ./target/release/popcorn-cli submissions list --leaderboard matmul_v2

ID       Leaderboard          File                 Time                 GPU(s)       Status          Score
---------------------------------------------------------------------------------------------------------

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/cmd/mod.rs	40.00%	9 Missing ⚠️

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