Revert "Split installation shell command into four"

This reverts commit bd24d0c.

It makes sense to revert the changes because every `run()` statement runs in its own container, thus it cannot be ruled out that other code is building the preinstall or postinstall commands under the assumption that they will run in the same container (or even in the same container as the conda commands). An example is available on #19545 (comment).

lib/galaxy/tool_util/deps/mulled/invfile.lua

@@ -72,20 +72,29 @@ else
 end
 
 local preinstall = VAR.PREINSTALL
+if preinstall ~= '' then
+    preinstall = preinstall .. ' && '
+end
 
 local postinstall = VAR.POSTINSTALL
+if postinstall ~= '' then
+    postinstall = '&&' .. postinstall
+end
 
 inv.task('build')
     .using(conda_image)
         .withHostConfig({binds = {"build:/data"}})
         .run('rm', '-rf', '/data/dist')
     .using(conda_image)
         .withHostConfig({binds = bind_args})
-        .run('/bin/sh', '-c', preinstall)
-        .run('/bin/sh', '-c', conda_bin .. ' create --quiet --yes -p /usr/local/env --copy')
-        .run('/bin/sh', '-c', conda_bin .. ' install ' .. channel_args .. ' ' .. target_args
-             .. ' --strict-channel-priority -p /usr/local/env --copy --yes ' .. verbose)
-        .run('/bin/sh', '-c', postinstall)
+        .run('/bin/sh', '-c', preinstall
+            .. conda_bin .. ' create --quiet --yes -p /usr/local/env --copy  && '
+            .. conda_bin .. ' install '
+            .. channel_args .. ' '
+            .. target_args
+            .. ' --strict-channel-priority -p /usr/local/env --copy --yes '
+            .. verbose
+            .. postinstall)
     .wrap('build/dist/env')
         .at('/usr/local')
         .inImage(destination_base_image)