chore: Modernize packaging scripts (#468)

- Adds shebang directive for the default shell (used when the directive
  is missing).
- Uses `set -euxo pipefail` to enable important shell options.
- Ignore shellcheck warnings related to missing `compile.env` source
  files.
- Removes unnecessary GOPATH export.
- Removes `-mod=vendor` flag from `go build` command, as that flag is on
  by default in recent versions of Go.

packages/log-cache-cf-auth-proxy/packaging

@@ -1,6 +1,8 @@
-set -ex
+#!/bin/bash
 
+set -e -u -x -o pipefail
+
+# shellcheck source=/dev/null
 source /var/vcap/packages/golang-1.22-linux/bosh/compile.env
-export GOPATH=/var/vcap
 
-go build -mod=vendor -o ${BOSH_INSTALL_TARGET}/log-cache-cf-auth-proxy ./cmd/cf-auth-proxy
+go build -o "${BOSH_INSTALL_TARGET}/log-cache-cf-auth-proxy" ./cmd/cf-auth-proxy

packages/log-cache-gateway/packaging

@@ -1,10 +1,12 @@
-set -ex
+#!/bin/bash
 
+set -e -u -x -o pipefail
+
+# shellcheck source=/dev/null
 source /var/vcap/packages/golang-1.22-linux/bosh/compile.env
-export GOPATH=/var/vcap
 
 VERSION=$(cat version)
-go build -mod=vendor \
-  -o ${BOSH_INSTALL_TARGET}/log-cache-gateway \
+go build \
+  -o "${BOSH_INSTALL_TARGET}/log-cache-gateway" \
   -ldflags "-X main.buildVersion=${VERSION}" \
   ./cmd/gateway

packages/log-cache-syslog-server/packaging

@@ -1,6 +1,8 @@
-set -ex
+#!/bin/bash
 
+set -e -u -x -o pipefail
+
+# shellcheck source=/dev/null
 source /var/vcap/packages/golang-1.22-linux/bosh/compile.env
-export GOPATH=/var/vcap
 
-go build -mod=vendor -o ${BOSH_INSTALL_TARGET}/log-cache-syslog-server ./cmd/syslog-server
+go build -o "${BOSH_INSTALL_TARGET}/log-cache-syslog-server" ./cmd/syslog-server

packages/log-cache/packaging

@@ -1,6 +1,8 @@
-set -ex
+#!/bin/bash
 
+set -e -u -x -o pipefail
+
+# shellcheck source=/dev/null
 source /var/vcap/packages/golang-1.22-linux/bosh/compile.env
-export GOPATH=/var/vcap
 
-go build -mod=vendor -o ${BOSH_INSTALL_TARGET}/log-cache ./cmd/log-cache
+go build -o "${BOSH_INSTALL_TARGET}/log-cache" ./cmd/log-cache