ghidra: derive the 7-Zip platform assertion from uname, not from my host

CI caught this on amd64, and the failure is the useful kind:

    sevenzip_native: grep -q 'PLATFORM=Linux-arm64' exit code 1
    stdout:
    PLATFORM=Linux-amd64
    ENTRY=SEVENZIP_PARITY_OK.txt

The test was wrong, not the package. I hardcoded the architecture I
happened to build on. Note what the same output proves: on amd64 the
native initialised AND decompressed the container — ENTRY is right there
— so 7z works on both arches and only the assertion was too specific.

Now derived from uname: Linux-arm64 on aarch64, Linux-amd64 on x86_64,
and an explicit failure on anything else rather than a silently skipped
assertion.

It still catches the regression it exists for. On arm64 the answer is
Linux-arm64 ONLY because build.sh swaps the all-platforms jar for the
per-platform one; if that regresses, the platform list stops being a
single entry, getPlatformBestMatch() falls back to matching os.arch
("aarch64") against "Linux-arm64", and cannot succeed — so an arm64 host
reports Linux-amd64 and the test fails. Verified that case explicitly:

  uname=x86_64   want=Linux-amd64  vs CI's real output   PASS
  uname=aarch64  want=Linux-arm64  vs our build          PASS
  uname=aarch64  serving Linux-amd64 (the regression)    FAIL

This is the first real feedback from the amd64 path, which I had flagged
as untested — the [ -f ] guard that takes upstream's natives on x86_64
now has evidence behind it too.

14/14 on arm64 after the change.

Author: bryan-minimal

packages/ghidra/build.ncl

@@ -208,7 +208,36 @@ let version = "12.1.2" in
           # ...and resolved to the ARM platform specifically. If the swap ever
           # regresses to shipping the all-platforms jar, the list stops being a
           # single entry and this is what notices.
-          ["/bin/bash", "-c", "grep -q 'PLATFORM=Linux-arm64' /build/7z.out"],
+          # ...and resolved to THIS HOST's platform. Deliberately derived from
+          # uname rather than hardcoded: the first version of this test
+          # asserted Linux-arm64 unconditionally and failed CI on amd64, where
+          # the correct answer is Linux-amd64 — the assertion encoded the
+          # architecture I happened to build on.
+          #
+          # Still catches the regression that matters. On arm64 the answer is
+          # Linux-arm64 ONLY because build.sh replaced the all-platforms jar
+          # with the per-platform one; if that swap regresses, the platform
+          # list stops being a single entry, getPlatformBestMatch() falls back
+          # to matching os.arch ("aarch64") against "Linux-arm64", and fails —
+          # so this reports Linux-amd64 or nothing on an arm64 host, and the
+          # test fails.
+          [
+            "/bin/bash",
+            "-c",
+            m%"
+              set -eu
+              case "$(uname -m)" in
+                aarch64 | arm64) want=Linux-arm64 ;;
+                x86_64) want=Linux-amd64 ;;
+                *) echo "unsupported arch $(uname -m)" >&2; exit 1 ;;
+              esac
+              grep -q "PLATFORM=$want" /build/7z.out || {
+                echo "expected PLATFORM=$want for $(uname -m); got:" >&2
+                cat /build/7z.out >&2
+                exit 1
+              }
+            "%
+          ],
           # ...and genuinely decompressed the container: the inner filename can
           # only appear if 7-Zip enumerated the archive.
           ["/bin/bash", "-c", "grep -q 'ENTRY=SEVENZIP_PARITY_OK.txt' /build/7z.out"],