Drop the redundant echo pipe that makes search-index.db fail with SIGPIPE

[thomasjm]

search-index.db intermittently fails to build with exit code 141 (128 + SIGPIPE):

error: Cannot build '/nix/store/...-search-index.db.drv'.
       Reason: builder failed with exit code 141

The builder is:

echo | sqlite3 $out <<- EOF
...
EOF

The pipe makes sqlite3's stdin the read end, but the heredoc then reassigns stdin to the temp file. Nothing ever reads the pipe, so echo writes to a pipe whose read end is already closed and takes SIGPIPE. Whether it dies is a race: if the write lands in the pipe buffer before sqlite3 drops the read end it survives, otherwise it doesn't. stdenv runs builders under set -o pipefail, so the pipeline reports echo's 141 and the build fails.

The echo has no effect on sqlite3 either way — the heredoc already supplies stdin — so removing it is the whole fix.

Reproduction, 3000 iterations:

for i in $(seq 1 3000); do
  bash -c 'set -eu -o pipefail; echo | cat > /dev/null <<- EOF
hello
EOF' 2>/dev/null || echo fail
done | wc -l

15 failures out of 3000 (~0.5%), each with exit code 141. With the echo | removed: 0 out of 3000.