Improve error messages of generate-test-{sxgs,wbns}.sh

`go get` installs commands in $GOPATH/bin (or ~/go/bin if $GOPATH is not
set), which may not be listed in the user's PATH environment variable.

crrev.com/c/2556739 updated third_party/blink/web_tests/http/tests/
loading/sxg/resources/generate-test-sxgs.sh, and this updates the
remaining scripts and READMEs.

Bug: 1151384
Change-Id: I2719fad30d3bb1141fb7abbecb8a2cf1b638a51d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2557159
Reviewed-by: Hayato Ito <[email protected]>
Commit-Queue: Kunihiko Sakamoto <[email protected]>
Cr-Commit-Position: refs/heads/master@{#830883}

Author: irori

signed-exchange/README.md

@@ -19,4 +19,5 @@ regenerate these files by running `generate-test-sxgs.sh` in the
 To install them, run:
 ```
 go get -u github.com/WICG/webpackage/go/signedexchange/cmd/...
+export PATH=$PATH:$(go env GOPATH)/bin
 ```

signed-exchange/resources/generate-test-sxgs.sh

@@ -19,6 +19,7 @@ for cmd in gen-signedexchange gen-certurl dump-signedexchange; do
     if ! command -v $cmd > /dev/null 2>&1; then
         echo "$cmd is not installed. Please run:"
         echo "  go get -u github.com/WICG/webpackage/go/signedexchange/cmd/..."
+        echo '  export PATH=$PATH:$(go env GOPATH)/bin'
         exit 1
     fi
 done

web-bundle/README.md

@@ -6,4 +6,5 @@ This directory contains tests for the
 To install them, run:
 ```
 go get -u github.com/WICG/webpackage/go/bundle/cmd/...
+export PATH=$PATH:$(go env GOPATH)/bin
 ```

web-bundle/resources/generate-test-wbns.sh

@@ -5,6 +5,7 @@ set -e
 if ! command -v gen-bundle > /dev/null 2>&1; then
     echo "gen-bundle is not installed. Please run:"
     echo "  go get -u github.com/WICG/webpackage/go/bundle/cmd/..."
+    echo '  export PATH=$PATH:$(go env GOPATH)/bin'
     exit 1
 fi