fix(e2e): replace bash-specific [[ with POSIX [ for OSTYPE check

[whg517]

Chainsaw uses /usr/bin/sh (dash). Replace [[ with [ and == with = for OSTYPE detection.

[Copilot]

Pull request overview

This PR updates a Chainsaw E2E test script to avoid bash-specific conditional syntax so it can run under /bin/sh (dash), specifically for choosing the correct date invocation on different OSes.

Changes:

• Replaces [[ ... ]] with POSIX [ tests.
• Replaces bash pattern matching (darwin*, linux-gnu*) with equality checks for OSTYPE values.

Comments suppressed due to low confidence (1)

test/e2e/chainsaw/pod-expires/chainsaw-test.yaml:24

• The new checks use exact string equality (e.g., OSTYPE = darwin / linux-gnu) but OSTYPE commonly includes suffixes (e.g., darwin20.0, linux-gnu with additional qualifiers), which the previous darwin* / linux-gnu* pattern handled. If you keep using OSTYPE, use a POSIX pattern match via case (or another portable approach) so macOS/Linux variants still match.

          if [ "$OSTYPE" = "linux-gnu" ] || [ "$OSTYPE" = "linux-musl" ]; then
            NOW_AFTER_5s=$(date -d "+5 seconds" -u +"%Y-%m-%dT%H:%M:%SZ")
          elif [ "$OSTYPE" = "darwin" ]; then
            NOW_AFTER_5s=$(date -v+5S -u +"%Y-%m-%dT%H:%M:%SZ")

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