fix(install): fix nightly digest extraction on macOS (#331)

## Summary

The nightly install script fails on macOS with:
```
gunzip: (stdin): unexpected end of file
```

The OCI manifest digest extraction in Step 4 used `sed 's/},{/}\\n{/g'`
to
split layers onto separate lines before awk extracted the digest. This
had
two bugs:

1. **BSD sed (macOS)** treats `\\n` as literal characters, not a newline
—
   the manifest stays on one line and awk always finds the config digest
(`sha256:44136fa...`, a 2-byte empty `{}` blob) instead of the binary.
2. **First-layer edge case**: the first layer shares a line with the
config
   block because the array starts with `[{`, not `},{`.

The result: downloading 2 bytes of `{}` instead of the ~23 MB compressed
binary.

## Fix

Replace the `sed` + `awk` pipeline with a single `awk` pass that tracks
the last-seen `"digest"` value and prints it when the target filename
matches in `"org.opencontainers.image.title"`. No `sed` needed, works
on both GNU and BSD awk.

Author: BYK

install

@@ -127,11 +127,18 @@ if [[ "$requested_version" == "nightly" ]]; then
 
   echo -e "${MUTED}Installing nightly sentry ${version}...${NC}"
 
-  # Step 4: Find the blob digest for this platform's .gz file
+  # Step 4: Find the blob digest for this platform's .gz file.
+  # Each OCI layer has "digest" before "org.opencontainers.image.title".
+  # Track the last-seen digest and print it when the target filename matches.
+  # This avoids sed newline replacement which differs between GNU and BSD.
   gz_filename="sentry-${os}-${arch}${suffix}.gz"
   digest=$(echo "$MANIFEST" \
-    | sed 's/},{/}\n{/g' \
-    | awk -F'"' "/\"${gz_filename//./\\.}"'/{for(i=1;i<=NF;i++) if($i=="digest"){print $(i+2);exit}}')
+    | awk -F'"' -v target="$gz_filename" '{
+        for(i=1;i<=NF;i++){
+          if($i=="digest") d=$(i+2)
+          if($i=="org.opencontainers.image.title"&&$(i+2)==target){print d;exit}
+        }
+      }')
   if [[ -z "$digest" ]]; then
     echo -e "${RED}No nightly build found for ${gz_filename}${NC}"
     exit 1