chore: Reference magma VM more explicitly

[jheidbrink]

Summary

Explicitly name the magma VM as we do for the other VMs.

Test Plan

Vagrant up/halt/ssh still works

[github-actions]

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[github-actions]

dp-workflow

0 tests   0 ✔️  0s ⏱️
0 suites  0 💤
0 files    0 ❌

Results for commit 0ac9f99.

[github-actions]

feg-workflow

    2 files  203 suites   40s ⏱️
374 tests 374 ✔️ 0 💤 0 ❌
388 runs  388 ✔️ 0 💤 0 ❌

Results for commit 0ac9f99.

[github-actions]

agw-workflow

615 tests   611 ✔️  3m 52s ⏱️
    2 suites      4 💤
    2 files        0 ❌

Results for commit 0ac9f99.