[install] handle bun add of existing peerDependencies correctly

[alexlamsl]

What does this PR do?

Previously bun add pkg on the following package.json:

{
  "peerDependencies": {
    "pkg": "^1.2.3"
  }
}

will result in failure, i.e.:

{
  "peerDependencies": {
    "pkg": ""
  }
}

without installing pkg.

• Documentation or TypeScript types (it's okay to leave the rest blank in this case)
• Code changes

How did you verify your code works?

• I checked the lifetime of memory allocated to verify it's (1) freed and (2) only freed when it should be
• I or my editor ran zig fmt on the changed files
• I included a test for the new code, or an existing test covers it
• JSValue used outside outside of the stack is either wrapped in a JSC.Strong or is JSValueProtect'ed

[Jarred-Sumner]

Thank you