CI: @iota/isc-sdk:test fails on PRs from external repos

## Bug description

```
❯ tests/L1ToL2Tokens.spec.ts (0 test)

⎯⎯⎯⎯⎯⎯ Failed Suites 1 ⎯⎯⎯⎯⎯⎯⎯

FAIL tests/L1ToL2Tokens.spec.ts [ tests/L1ToL2Tokens.spec.ts ]
Error: Failed to parse IOTA EVM Bridge config JSON env var! Unexpected end of JSON input
❯ loadConfig tests/config.ts:56:15
54| evmBridgeConfig = JSON.parse(rawEvmBridgeConfig);
55| } catch (error) {
56| throw new Error(
| ^
57| `Failed to parse IOTA EVM Bridge config JSON env var! ${(e…
58| );
```
For example https://github.com/iotaledger/iota/actions/runs/15969070816/job/45036051562?pr=6777

Probably caused by the usage of secrets here https://github.com/iotaledger/iota/actions/runs/15969070816/workflow?pr=6777#L98-L99